Perhaps instead of an imperial unit to replace the regular civ's troops, how about an imperial promotion - +100% exp gained inside cultural borders or some such and perhaps a unit cost as well (1 gold per unit maybe?)
Also, for those challenging the new imperial union, you may want to give them a rebel promotion or some such. 99% of the time, the civ declaring the Union is going to have a major military advantage anyways. No player is gonna delcare all out war unless they have it. So perhaps allow rebel units to come into play (cheaper cost, 100% exp gains per combat, and reduced maintenance) to give the declaring player the challenge that is needed.
If the rebels win, the rebel and imperial promotions will need to be lost and the game go on as regular -- OR -- if another civ captures the imperial palace, they now become the imperials and the promotions would have to switch. The victory counter begins anew and the battle rages on.
Now, since I have no idea how to mod, program, etc, I have no idea what kind of work this would need. Again, just throwing ideas out there for y'all.
Also, for those challenging the new imperial union, you may want to give them a rebel promotion or some such. 99% of the time, the civ declaring the Union is going to have a major military advantage anyways. No player is gonna delcare all out war unless they have it. So perhaps allow rebel units to come into play (cheaper cost, 100% exp gains per combat, and reduced maintenance) to give the declaring player the challenge that is needed.
If the rebels win, the rebel and imperial promotions will need to be lost and the game go on as regular -- OR -- if another civ captures the imperial palace, they now become the imperials and the promotions would have to switch. The victory counter begins anew and the battle rages on.
Now, since I have no idea how to mod, program, etc, I have no idea what kind of work this would need. Again, just throwing ideas out there for y'all.