Instead of instituting declines, a different approach would be to make it less "profitable" from a gameplay standpoint to annex everything in sight. I believe there is a value that could be achieved by making allies/vassals/satellites worthwhile to have around in the game, worth more than just outright annexing. If this is achieved, then it naturally becomes an obvious block to growth. The rebels from newly conquered provinces are just annoying, and let's be honest, if you have a large standing army, you just run it around crushing the rebels anyways. Ultimately the tools you use to expand are the same tools that you use to keep expansion profitable.
I think the CB system in Victoria 2 isn't a bad model for some of this. Use a system like this for diplomacy/national interactions. It will slow things down, but at the same time, make alliances/unions last longer and take efforts from other countries to drive that wedge apart. Part of the reason we annex countries in EU was that you could never really count on them to do anything useful for any length of time, not to mention we could manage that country better than the AI anyways. If we strengthen the diplomatic part of the game and make alliances useful and long lasting, we probably will be less tempted to annex them the first time they break our alliace for no reason.
You could also put a large penalty on the rewards of newly conquered territory and then make the player use alot of resources to remove the penalty. Make production, tax, and manpower different penalties so players can pick and chose were to invest the resources. Instead of playing whack-a-mole with rebels, make it just another large dissention penalty that is only slowly reduced if you actively occupy the province with a certain number of soldiers. Annexing five or six provinces may seem like a good idea, until I realize that I will need to tie up 50k troops for 50 years to make those places actually useful to me.
I think the CB system in Victoria 2 isn't a bad model for some of this. Use a system like this for diplomacy/national interactions. It will slow things down, but at the same time, make alliances/unions last longer and take efforts from other countries to drive that wedge apart. Part of the reason we annex countries in EU was that you could never really count on them to do anything useful for any length of time, not to mention we could manage that country better than the AI anyways. If we strengthen the diplomatic part of the game and make alliances useful and long lasting, we probably will be less tempted to annex them the first time they break our alliace for no reason.
You could also put a large penalty on the rewards of newly conquered territory and then make the player use alot of resources to remove the penalty. Make production, tax, and manpower different penalties so players can pick and chose were to invest the resources. Instead of playing whack-a-mole with rebels, make it just another large dissention penalty that is only slowly reduced if you actively occupy the province with a certain number of soldiers. Annexing five or six provinces may seem like a good idea, until I realize that I will need to tie up 50k troops for 50 years to make those places actually useful to me.