I think it would be interesting if, by event, every now and then a pretender to the leadership of another nation came knocking at your door looking for allies. It wouldn't have to be a pretender to the throne type deal, it might be someone with popular support but who lost an election in a republic, for example, and was forced to flee, or someone similar to Ptolemy Keraunos, who didn't really have a claim to the throne of Macedon but was able to manipulate Seleucus into opening the way for him anyway (and then murdering Seleucus). This could give you three options: turn the traitor over to the other nation for an opinion boost with them, offer them shelter and minor support for an opinion penalty and maybe a bit of legacy (and perhaps making any civil wars in the near future a bit tougher for them), or throwing full support behind the pretender and immediately declaring war.
If you go the latter route and manage to occupy the enemy capital, you could get another set of options--install the pretender and go home, probably leaving you with an alliance or cooperation treaty at least, install the pretender as a client (chance for them to accept, with relations souring if they do not, or maybe only allowable to attempt at a certain power disparity), or betray the pretender and take their lands yourself (which may have a penalty to unrest there for a while).
I haven't had a chance to play the game yet (got it installed last night but a pesky little thing called "work" means I won't be able to actually play until this evening) so I don't know if this kind of mechanic already exists in some form (if so--great
) I also don't know if the peace system would allow for that--perhaps something to consider as the diplomacy side gets fleshed out a bit? And if the concern is that this kind of thing might upset the balance of multiplayer too much when possibly used on one of your rivals or wouldn't be fun to play against, perhaps set the event so that it can only fire vs. the AI?In any case, I'm super psyched to try out Empires. Once I have a bit of experience under my belt I may even try modding something like this in.