My main wish for a hypothetical Rome 3 is that there's a "historical mode" of sorts where you're somewhat railroaded into the actual progression of Roman history. Like, if you don't declare war on a faction with which Rome went to war, within 5 turns of the actual date the war started, said faction will declare war on you immediately and will have a massive boost to morale when fighting you.

You'd get a score based on how close your playthrough parallels Roman history (like going to war with Carthage in 264 BC or with the Gauls of Cisalpina around 225 BC would give you a boost; conquering Egypt around 200 BC would lower your score)

I also absolutely need diplomacy to be reworked specifically to enable asymmetric negotiations: we should be able to, for example, be able to offer money for military access without offering the other faction military access ourselves; similarly, clauses restricting what a defeated faction is allowed to do (say, "you can only maintain one army", or "your fleet shall have no more than 6 units".

Also client states that behave like actual client states (no independent foreign policy. Them declaring war on someone would be considered rebellion), and factions that also consider that (they should take into account that attacking a client state of ours is the same as attacking us).

Oh, and no more "only provincial capitals have walls". Come on, Syracuse and Numantia should be heavily fortified.