i want to see this mod happen, i expect there would be some buildings in the game that can be modded into rome era ones [e.g. constantinople]. there will be some battle ai mechanics as said earlier, and if formations are moddable i will definitely be modding empires, this was sadly missing from rtw.
basically it all comes down to madding files, will we have any? if we do they just need to be comprehensive so we can change it all around.
i would suggest that a big team be built for mods of this era, then variation of that can be made individually for earlier and later eras. if we don’t do something like that it will be years before we get any mods and CA may even beat us too it with there own rome2 [after napoleon presumably].
i don’t think we will even get modding files to begin with, maybe not at all.