Main concepts
I was reading though workshop threads, and it the unit creation received the most attention through the years. General aspects were touched, but no decisions were made. Now, when the units are finished, I can think about them
Story and the scale
As it was intended, the mod will be set on some random planet N, where different factions gathered to fight each other. I will borrow ideas from Dawn of War games here, and say, that they are fighting over remains of Imperator Class titan, contruction of which will replace the Space Race. Some factions may want to get it (Chaos, Imperium), other may want to destroy it (Eldar). DoW also has answers for reasons to fight. Imperial factons do not necessary have centralized command and their orders may contradict. Inquisition can purge Space Marines, if they suspect heresy, Dark Angels may search for Fallen, Imperial Guard may fall to heresy, etc. Eldar and Tau will be single factions, so they will never fight allies. Chaos and Orks are easy - chaos warbands often fight each other for glory, for example. Orks don't care who they fight at all.
The starting unit, which will replace settler, for all faction will be the Webway Gate. Once again, borrowing from Dawn of War, the system got caught in a Warpstorm, which prevents normal space travel. They found an abandoned webway enterance on neighbour planet, though and could safely travel through it to the starting points.Of course, WWG would be unbuildable and required only for authentic start, since it is impossible to give faction-specific settlers as the starting units.
For settling, I will adapt CCM's systems of settler spawning. I am not a fan of early game settler rush and it is completely not suitable for WH40K. It is only fitting theat high command sends base-builders from time to time, when positions are observed and secured. It also make early wars more profitable, and the whole concept of 40k universe is meaningless, merciless war without end. I will try to make factions as aggressive as possible and recommend to play only on "Most Agressive" settings.
Factions
Five "races" - Imperium, Chaos, Orks, Eldar and Tau. Eldar and Tau will be represented as singular races. I knew, there were plans of diving Eldar in different craftworlds, but unlike chapters of marines and legions of chaos, craftworld differences are barely represented on tabletop. While Orks are in the same situation, aaglo has made all units for them and they really have a lot of character. Leaderheads for orks are also ready, and eldar have none. Finally, orks are numerous and often fight each other, while Eldar rarely have inner conflicts and work together to save their dying race.
Each race will have their unique government, except the Imperium - Space Marines have quite different organization and will have unique government.
Imperium
Includes Imperial Guard, Witch Hunters and Daemon Hunters (yes, I prefere the old names). Their focus are strong infrastructure, economics, happiness, but their units can be mediocre or too expensive. They also suffer from corruption and do not have espionage. The idea is to make their early and mid-game hard, but as they gain power, they can roll over anything (standard civ3 experience, lol).
Space Marines
They will have unique Chapter organization. Corruption is minimall, but they may have problems with income and happiness. To replicate Chapter organization restrictions, they will have high free units support number, but don't get much from additional towns, and cities/megas won't give military support bonuses at all. Support cost will be high, so going over optimal number of units is highly restrictive. This hopefully simulate the situation, when you can have limited number of very strong units. To compensate this, Space Marines will have access to cheap servitor/automated defence systems, which can protect captured cities/serve as military police.
Chaos
Chaos will have a lot of problems to deal with. Their command is often decentralized, warlods want to take over, plot schemes, etc. etc. They will have high corruption, reduced worker rate, maybe limited research rate. But their main advantage is high cultural rate. I guess, they will be the easiest race to achieve a cultural victory. They will have more cultural buildings and esnlave sacrificable slaves with several units.They won't be xenophobic, like eldar, orks and impeium. They will also be the best spies in the game, maybe even Elite spies. Finally, their unit support will be more standard with less free units from the start, but more as they gain territory, while their units are almost as good, as Loyal Space Marines.
I also plan to dedicate each faction to chaos god and give unique buildings to represent it. For exapmple, Khornite Shrine will give barracks effect way earlier, before one can build barracks, while Slaanesh Shrine will increase happy faces. Slaves can be either sacrifices for culture, or be upgraded to Daemon units, to represent them being sacrifices to summon Daemons.
Orks
Orks are all about war. They won't have to worry about unit support at all, but corruption will be higher than for any other faction. To compensate this, they will have several "Forbidden Palace" style wonders. They will also have highest military police and draft limits, but lower worker rate and research cap. Maybe, they will produce settlers faster, to represent their fast growth or even get access to building settlers at some point.
Eldar
Eldar, just like on tabletop, will be tricky to play. They will be the only fation suffering from war weariness. They will have the lowest corruption, though. Their true strength will lie in their fast units (all infantry can move 2 squares, all vehicles are all terrain as roads and can paradrop) and auto-production units through Aspect Shrines. Their Avatar of Khaine leader unit will be strongest among other factions and can turn the tables early in the game, if generated. In short, I want eldar to avoid long wars and aim for short, victorious engagements, outmaneuvering their enemies and bringing them down with surgical strikes.
Tau
There are too many jokes about them being communists in 40k, so I can't help giving them Communal corruption. Like Chaos, Tau won't be xenophobic, letting them generate culture earlier in captured cities. They also will be the best diplomats in the game. Also, no other faction will hate Tau Government, and they will have easier time assimilating captured civilians and quelling resistance.