I'm considering only the base game—specifically the Atlas map at the "Giant" size—but we can still remove the map's circular boundary, which gains us a good amount of extra surface area; I don't think we lose much compared to *C&C Generals*. I've never tried it, but I believe map size is a limitation related more to performance than an actual limit of the engine itself.

Do you have any specific information about this? Actually, I’ve created maps larger than the ones in the game, but I haven’t tried playing on them yet because they were made for Hyrule Revival.

There are certainly plenty of challenges, but I still want to give it a try. The map is actually the aspect I'm least comfortable with; in reality, I could create maps many times larger than "gigantic," but crafting the terrain for them is truly exhausting, and it seems to impact performance as well.

Creating mods about exactly that, advancing and developing things normally within the limits of what is expected in a game, besides that I disagree about map sizes, normally the maps in 0ad don't go to maximum capacity of the engine. Wait and see, don't say something is impossible unless you've already tried it and seen that it really isn't possible. In that case, it would be interesting to hear about their experiences and gain some knowledge from them.

I dont think 0AD engine is well suited for 21st century warfare. Maps are too small and 21st warfare is way more complex than 0AD warfare. You'd have to add drones (both recon and suicide) that means implementing some degree of AI behaviour + jamming mechanics + an implementation of data-link. Weapons like heavy artillery (CEASAR, HIMARS) would be : 1 very hard to balance, 2 not well suited for such smalls maps (portable mortar would be ok i guess). Also you'd have to think about AA defences, radar mechanics, MANPADS, and "modelling of missile behavior" --> implementing some simplified state-space model + control laws like in War Thunder. etc etc...   Maybe what you can do is focus on late 20th / early 21st century asymmetrical warfare like the French war in Mali ? Or do a CQB based mod (which implies huge assets developpement) like 6 Days in Fallujah