Concerning the "Pax Galctica" scenario:the pic below shows what I'm running into in seeing what I can do to generate the scenario. I can build the cities, but I don't get the option to selct the SDI or ODPs (and I remember now that this is controlled via Lua, so these items are not "buildings" or "units" per se). I can give the individual players the money to purchase these items, however I don't have that option with the C-S unfortunately. Rather frustrating!
Yes, because they're Projects you'd have to do it through Lua. At the start of the game, have a simple Lua function give everyone one copy of SDI and/or ten ODPs. I'm honestly not sure what this'd do for City-States, who are normally banned from all Projects; I'm trying to implement something for them right now.
The other option is for you to create a new building in XML. The nuke interception chance is handled entirely through Lua, so it'd be trivial for you to modify the Lua logic to recognize a building type as adding to interception chance; the only catch is that it'd have to loop over all cities, whereas Projects can be directly counted from the Players structure.
In fact, you wouldn't even need a building. In the Lua code I set the minimum and maximum chance of interceptions; you could simply remove SDI from the game, and set the minimum interception chance to 50%. Currently, it's 30% for A-bombs, 20% for nukes, 10% for planet busters, and 0% for a subspace generator. (You'd need to modify the logic to ensure the Subspace Generator isn't destroyed when its shot is intercepted.) Yes, I know this means you'll be missing out on some bits of artwork and such, and it'll make certain techs less desirable, but you can do it.
(Note that ODPs aren't just nuke interception, they also reduce the effects of orbital weapons through another Lua override. So if you remove their anti-nuke effect, you should increase the other.)
If you don't want to destroy the projects and want to still have a very nuke-unfriendly world, you could always just boost those minimum percentages by a flat amount; instead of 30/20/10, make it 60/40/20. You'd probably hit the maximum caps very quickly, though, as those are 83.3/66.7/50%.
Regardless, you can see the approach I am taking regarding the cities I plan on using for this scenario in that I am making them inaccessible to being conquered (land equivalents would be surrounded by mountains). And placing a nice range 4 Plasma Artillery in each city should condition the players to steering clear of these "cities" as well.
If you want them to be unconquerable without the inaccessibility bit, you can just give the cities a custom building that adds a huge amount to their Defense value; the unfortunate downside is that those cities' base ranged strike would then be incredibly powerful, even if its range would still be 2. That can be adjusted through Policies, though.
And remember: just because a city is surrounded by mountains doesn't mean it's actually inaccessible; quite a few units in my mod can move freely over mountains. Skimmers, Gravtanks, Vertols, Chiron Locusts, Nessus Worms, Formers, Combat Mechs, Labor Mechs, Bolos, and Gravships. Plus the orbital weapons can hit the cities, even if they can't capture it.
Modifying the xml files that control what the Barbs can build (i.e. a mod to your mod in a sense) I think should make them more challenging in the fixed environment that is a scenario (i.e. I would build the scenario around the strengths of my villain).
The problem there is that, as I mentioned, Barbarian camps just stop spawning once you reach a point in the game where the map is explored. If you want to keep Barbarians a threat after that, you MUST use a Lua override like my Spore Tower logic; since it's Lua, you can easily tweak it to spawn whatever you want, really.
Another idea that I had for a scenario was "Ice, Steel, and Water" which would take place on an icy water world. The map would have free-floating ice even in the equator region, and I'd have to edit the units to emphasize naval warfare ( I was thinking about sort of modeling the unit selection after Schafer's "Final Frontier", which to a certain extent can be viewed as similar if you just replace space with water, and solar systems with archapeligos).
You might want to consider looking into the book "Seas of Venus" by David Drake, downloadable
here. (It's free to download if you use the links on the left side of that page.) Obviously not ice-based, and there's no mention of submarines, but it's all about a waterworld where the little bits of land are basically uninhabitable (due to incredibly powerful native wildlife) and everyone lives in undersea domes that are more-or-less immune to attack, with "wars" basically being organized skirmishes between mercenary forces where the loser capitulates immediately.
The biggest problem for any of this is that with increased naval emphasis, you'll just need more units; part of the reason for the Stealth Ship and Leviathan in my mod is that I was trying to REDUCE the number of types of naval units, since they became less and less important as time went on. With long-range air units (and eventually orbitals) taking over the bombardment duties (and having more than enough range to hit targets without needing to base on a vulnerable carrier), Vertols and such being all-terrain units, and most land units now having more than enough mobility, I was trying to close down the naval operations.
You'd basically need to do the opposite, creating an even larger number of naval units to fill the roles that many of the land units filled. Or, modify some of the existing ones; the Skimmer could be given the Vertol's all-terrain promotion, to allow it to fill the mobile defensive infantry role while the vertol remains the offensive tool, although you'd want to reduce the Skimmer's other stats to compensate. The Chiron Locusts could easily be given this ability as well, as-is.
You'd want some new naval unit, to replace things like the Gravtank; it could be sort of a Battleship/Submarine hybrid, to balance the Leviathan. Give it some missile racks as well, instead of fighters, and remove the missiles from the Stealth Ship; you'd now have three naval units that each have their own pros and cons. Likewise, you'd need to modify the Bolo and Combat Mech (the only two Titans without the all-terrain ability) to be naval units.
One thing I noticed when I was playing around with the map generator for waterworlds was that there wasn't very much uranium placed naturally in waterworld style maps. Not a problem in a scenario, but I wonder how players who enjoy waterworld environments deal with it?
There are basically three options for this:
1> Remove Uranium as a resource. You don't have to actually remove the resource itself, just make any units that require it use, say, Oil instead. So less "nuclear sub" and more "modern sub", like how the Modern Armor replaces the Tank. Maybe have it require Aluminum as well.
2> Adjust AssignStartingPlots (and a few bits of XML) to spawn water-based Uranium. You'd need to expand what I went through to get the AI to improve its own water-based Omnicytes and Dilithium, to include this new resource.
3> Use the Building Resources mod component (which is already included in my mod) to create buildings within each city that generate the land-based resources. So if each city gets a free building that generates 1 unit of Uranium automatically, then there'd be no need to have large numbers of deposits on the map. You don't even need to create a new building for this; just give it to an existing building that every city gets automatically, through one simple XML declaration.
The thing is, on waterworld maps you'd be short on all the other resources as well, especially Aluminum. If you're sticking with only futuristic settings then you can ignore Iron, Horses, and Coal; Oil will be present in the water, but unless you change the Lua it'll be much more spread out than in the vanilla game. That leaves Aluminum and Uranium. So option #1 above isn't really practical, but the other two can be expanded to include both resources.
The real problem, as always, is the AI. If you make a map that requires a very specific playstyle, the AI just won't adapt well to the changes. In a waterworld, naval bonuses become very important, but the AI's flavor values don't really change so it wouldn't be any more likely to build these as before... unless you change those as well. And if the city in question is on a 1-hex island, the AI will continue to build land units there even if it has nowhere to put them.