All cities would probably have 1 ring (maybe 2 rings for certain civs, like Tau and the Imperium)
Tau and Eldar would be the late game civs ... while Orks would be the early game rushers.
(Orks need less tech for spacetravel, but obviously don't do it as well)
Tanks, Titans, and other Seige equipment should probably have a chance of being captured. Well, capturing a Titan would probably tricky as all hell/ next to impossible, but maybe with enough techpriests .... (I'd say Orks, Marines, Chaos, Imperium, Sisters of Battle, Tau, Eldar -> All races that can capture other's 'machine tech'). Maybe Orks should be limited to capturing tanks and spaceships.
As far as that goes, perhaps Tyrannid should be able to capture spaceships (derelects?)
Tao/Eldar -> science
Eldar -> teleportation
Orks -> food + MFG (super efficient whips and drafts)
Imperium -> best maintenance (Organized Traitx1.5, Courthouse UB), best overall armada
-> lighthouse turns into "space dock" -> Their spacedock UB should give faster ship production similar to Drydock ... they have the 3rdor4th best ships in the Galaxy and can produce the LARGEST fleet (other than perhaps Tyrannid of course)
and while Imperium fleets are the largest, Eldar fleets are the fastest, Tau fleets are the strongest, and Chaos fleets are almost fast as the Eldar and almost strong as the Tau
-> And of course the
mongolian horde Tyrannid fleet can get insane numbers, but is the slowest (slightly slower than Ork fleet)
Come to think of it ... While the Tyrannids should not be able to build more than 1 Hive ship at a time (national limit of 1) they should be able to "make more" through combat ... but only via hive ship. For example. If a Hive ship eats another ship, there is a chance to create a new Hive ship. This is the only way (playable)Tyrannids can get more than 1 hive at a time: by using their Hive ship in combat.
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now onto land combat
All Ork units are pretty weak (except for Nobs and Warlord). Orc Warchief should probably have 2nd highest strength rating (just below Necron and just above Tyrannid Tyrant) ... with all other commanders about 40% the strength of a Tyrannid Tyrant. Correct me if I am wrong of course, but I feel this would be the coolest

.
Orks are highly cost effective, but not necessarily "strong". Ork grunt is 80% strength of Imperium Guardsman, and 40-50% strength of Necron grunt (possibly). But about 40-60% the cost of Imperium Guardsman, and no maintenance costs.
not sure if there should be a difference between melee and ranged combat ... but if so, then most ranged combat should just be "1 tile" while most artillery should be "2 tiles". (With Eldar + Imperium at 3 and Tau at 4 -> other than Imperium Basilisk cannon which is also 4 range).
Spaceships should be immune to damage from artillery (as they are ... in space).
Imperium tanks should cost perhaps 3 extra gold per turn, but be easily the best land units in the game (other than Titans and Leviathan type Tyrannid). Tanks should probably have just 1 tile range, with the exception of their super-tank which gets 2 range. As far as strength rating goes ... possibly the size of 2 necron commanders for regular tanks, and 3 necron commanders for the supertank. As a point of reference, the Bloodthirster should be about the strength of a necron commander in this meta-mod. (yes, I know, but aren't they gods? ;p On a serious note, the extra strength of the Necron Commander is to reflect their ability to, um, become temporarily invincible xD -> Sister's of Battle 'super angel' should likely be about the same, if not a bit stronger/andor bonus damage to Chaos units)
Big Picture: Imperium Guardsmen should be the 3rd weakest unit in the game (above Ork grunt, and Tyrannid grunt), while at the same time the least cost effective unit at face value (more expensive than Necron grunt and Space Marine scout) ... but ultimately, considering a mid game to late game Imperium super city, can easily *become* the most cost effective unit in the game. Only 'civilized' unit that causes 0 war weariness
Titans should get 2 range on their super gun, and perhaps have a strength rating of 5 NCs (necron commanders).
meanwhile the Tau have the best end-game units, and throughout the entire tech tree have the best Artillery and Ships.
Chaos Marines: Heretics should get thrown out (chaos fodder units from Dawn of War), and instead their non-marine grunts should become the chaos demons. AKA: Lesser demons should spawn from Warpgates (Planar gate + Obsidian gate rolled into one).
Nice effect of the warp gates the chaos uses ... is that any city on any planet can gate one unit to any other city on any other planet. (sort of like airport lifting over the ocean in Civ IV, only its across "space")
Space Marines: deep strike units get Commando (or treat all land as roads, or will para-drop, or fly like helicopters ... will depend on game mechanics)
Cheapest unit is the scout. Cheaper than Imperial Guardsman but more expensive than Necron Grunt. Relatively weak, somewhat fast, can see invisible units. Can upgrade into "Scout-Assassins" or something. (Space Marine equivalent to the Imperium's vindicare assassins). Yes, normally Scouts can equip sniper rifles etc, but I think separating the unit into two different units is best for now.
Regular marines: Rocket marine (AT infantry), Marines (good vs melee) and Vet Marines (good vs melee and laser inf)
Terminator Marines -> Flanking vs Seige weapons? (some sort of marine should likely have siege flanking) -> Other than that, 2 types, 1 good vs melee, the other good vs vehicles and seige.
Most units should be fairly expensive, but fairly strong as well. Maybe able to take more promotions a la the "great general" promotions of Civ IV. Or all start with the leadership promotion, etc.
Eldar should focus on speed and withdrawal. Most vehicles should be fairly cost effective, but infantry not very cost effective. Relying more on special gimmicks (spells, teleportation, withdrawal, first strikes, healing, illusions, etc) to stay alive. For instance, Alazkan's black mirror spell (harlequins?), high priest heal spell, disease and cure disease spells, escape spell, psionic blast/ring of fire, etc.
Most good Eldar units should likely have a national limit. Yes, all races will have some national limit units (like the commanders), but even Vehicles and Ships should have a nat limit for this race. Fastest non-warp technology. Not sure how to get chaos units to warp unless they can dock in port cities, but yea, Eldar ships have fastest 'natural speed', and therefore can redistribute their military the 2nd fastest (second only to Chaos warp gates).
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now onto the Cities
Imperium cities can get the highest population (with possible exception of fallow Tyrannids), but take awhile to get there. Can get the best late-game cities, (consider late-game civ with factories hooked up, and this is Imperium late game cities). 2 ring cities
Ork cities grow fast, max out fast, and are best used for whipping/drafting. 1 tile cities.
Consider somewhere between classical whip cities and Renaissance draft cities, but better at either. -> Possibly no anger from whips and drafts.
not a lot of buildings to choose from. new buildings are pretty scattered throughout the tech tree. Best strategy is just to pump the units and keep them coming, until your economy is deep in the red. (likely there should be 1 land unit that is "free" in maintenance, allowing a potential infinite unit supply).
An Imperium military industrial complex (1 super city) should likely be able to build a bigger, more cost effective army than any 3 orcish cities combined (all the factories and such). However, those 3 orcish cities can get started earlier, and the Imperium has to pay for each unit, while orks get grunts for "free".
Imperium tanks should cost perhaps 3 extra gold per turn, but be easily the best units in the game (other than Titans and Leviathan type Tyrannid)
meanwhile the Tau have the best end-game units, and throughout the entire tech tree have the best Artillery and Ships.
Space Marines, Chaos Marines, and Eldar should have "combat centers" rather than cities, which means 1 ring instead of 2. (note: Orks, Necrons, and Tyrannid also have 1-ring cities, but for entirely different reasons)
Now, of these three, the Eldar have the highest GNP (especially science), while the Space marines have the highest MFG. Chaos is more balanced between MFG and Science (but not a lot of "gold producing" buildings, unlike the marines)
Tyrannid great people replaced by "Superspores/biospores" which can be used to bulb techs. (tyrannid should likely have a separate tech tree, or at least different enough functionality that it doesn't matter-> focused on evolution rather than technology)
Space marines should likely only have Great Generals and Great Engineers, while the Eldar have Engineers and Prophets, and the Chaos have Scientists and Generals. Chaos can also have access to Prophets and Engineers, but not as many as either Eldar or Space marines (obviously the generals can also be produced through combat, as opposed to GPP). Eldar engineers produce more science than normal engineers, and their prophets produce a bit of science as well. Net result is eldar can produce more science than Chaos marines.
Imperium and Tau have sizable access to all specialist types (other than artists that don't exist -> replaced by Generals), but their only Great General GPP sources come from one mid-game building each. The military academy and Warrior Shrine respectively.
Upon city capture, most factions gain gold based on building hammers as normal ...
But Tyrannids are different. While still gaining gold as a function of building hammers, they also gain Technology and Spores as a function of enemy population. They gain bonus research and population (in addition to the base value, likely triple the base value) if they choose to raze the city.
Most races can only have 1 commander each. But Imperium can get 2 commanders and the Tyrannid can have 4 hive tyrants.
-> if possible, all current hive tyrants should die if the last hive ship is killed
-> Imperium should likely have 4 lieutenants each, boosted by their particular Commander's promotions (using the Rife great general mechanic). The boost should occur regardless of distance between lieutenant and commander, or if there is a distance, it should be the diameter of a planet (aka must be on the same planet).
All Imperium officer units (2 commanders and 8 lieutenants, plus units with the great general promotion if possible) should produce double great general points if possible.
As i've said earlier, regular spores (through combat) can either add population to a city (or hive ship), be upgraded to units, or instantly add certain buildings to a city (or hive ship).
Super Spores (through GPP points) can be bulbed for a LOT of technology points, and can also be used to create additional hive tyrants?
The Hive Ship produces +100% GPP points, for the purpose of easier Super Spores when starting AND centralized rule (population better spent added to the Hive Ship in most cases). If a person has 2 hive ships, they can "merge" the two together ... basically by having the 1 hive ship "add" to the other to create a building. This building is a World Project otherwise known as "Birthright regained", but instead of being able to re-use your World Spell, the Tyrannids will be able to use their worldspell for the first time ever. This spell is otherwise known as "ardor" and will reset the GPP setting to 0, allowing for MANY new Super Spores.
I am thinking that Super Spores (and, by extension, specialists) may be the primary source of Science for the Tyrannids. 90% of their economy/buildings will be focusing on MFG and occasionally gold/maintenance.
A tyrannid's economy may not have natural inflation, but instead their army size inflation may eventually require them to either continue to capture new cities (for the capture gold) or be forced to delete some of their units in order to curb the expenses (we will abstract these expenses to mean the Tyrannid's feasting on the total natural resources they have aquired). -> And deleting these units is abstracted to parts of the army dying out as they begin to starve.
Tyrannids would naturally have spy units, with no actual combat abilities, namely the hybrids or whatever you call them. These spy units can be used to cause "city revolt" when the fleet grows near, in order to establish a beach head (Tyrannid have no natural orbital bombardment capabilities, so city revolts make invasions much easier). -> I mean, of course they could bombard the city with cannon fodder ... the Hive has a cargo of 20 and each ship it creates as a cargo of 5-10 depending, and newly created units from the hive are deposited into any one of these cargo holds ... but that is a LOT of hammers wasted. Better to send an invisible spy on an invisible ship, send it to the planet ahead of time ... and when the time is right, use a city revolt.
Inquisitors are the only units that can see and eliminate such spies. Races that have an inquisitor in some form (Imperium, Sisters of Battle, Space Marines, Chaos Marines, Eldar, Tao, and Necron)
For Tao, Necron, and Eldar, the ONLY inquisitor unit is the commander, making this an extremely hard form of infiltration to detect. Only high-level demons of the Chaos legion can detect it (a commander that has leveled up enough, and decided to evolve into a demon prince ... in order to give a new commander a chance to prove himself) -> commanders (nat limit 1) can upgrade into demon princes (no limit) at level 6. Also, commanders can turn themselves into a Bloodthirster (same unit, different art and new promos-> via spell instead of upgrade), and actual demon lords have a very low chance of spawning in via warp gate if the right buildings have been built. In this case, Abaddon the Destroyer counts as a demon lord.
So to recap, Necron, Tau, and Eldar commanders (1 each), and Demon Princes, Demon Lords, and Bloodthirsters are the "Inquisitors" for non-humans.
Meanwhile, regular inquisitors, which can be built normally, are the inquisitors for the three Human factions. (While Grey Knights get bonus vs demons and tyrannid, they do not count as inquisitors for this purpose). The "Inquisitor" commander type (default for Sisters, optional for Imperium and Marines) also counts as an Inquisitor.
The only other invisible unit is the corrupter/or whatever you wanna call it. This is an invisible missionary for the Chaos clan, which also uses an invisible ship (these invisible ships cannot EVER be detected, and have 0 strength, by the way), can then be let off in rival territory (<bmoverival>1<bmoverival>) and can spread the Chaos religion.
Chaos religion causes ... BAD ASS THINGS ... to happen to non-chaos cities. Think Muris Clan goblin events x10 ... mixed in with some appocalyptic horsemen 'maybe' appearing.
Most races have 2 options of dealing with this:
1) send in an inquisitor
2) exterminatus (abandon the city)
first option takes a hell of a lot longer, especially if you have only one inquisitor and he is half way across the galaxy ... but the second option gives a global happiness penalty. (possibly temporary, but certainly long lasting). Interesting decisions
The Eldar should have a rather crippling (but fortunate) civic that prevents foreign religion spread. This would be a good thing to do once one or two of your cities have turned chaos, in order to have time for inquisitor to visit all affected cities and cure them.
The Tao have a reduced chance for missionary events to succeed in all cities, which is sort of a soft cap against this tactic.
Human races obviously have an ability to produce multiple inquisitors, so they get no "special defense" against this tactic, other than the obvious fact that they have access to the only hard counter in the game (an inquisitor in every city) ... but of course this can be a sizable hammer investment ...
Honestly, the worst Chaos event is probably not a barbarian Demon Lord showing up, but instead for the city to flip to the Chaos faction. At least, if its a bad-ass city that would be BAD if it fell into the wrong hands ...
In scenarios where the Imperium is this MASSIVE BEHEMOTH of a neutral faction, their AI should likely have Exterminatus as the default response (5 turns after infection, in case there is an inquisitor nearby with "remove religions" AI turned on)