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