It feels like the behavior change you seek needs some sort of in-game incentive (due to the downsides of that change mentioned by others here).
For example, alliances choose government type. If they choose the one with those additional branches, then they get an income benefit. In exchange, they have a periodic in-game voting for those branches, and they can only declare wars if all branches click the “approve” button in-game.
Even after making voting in-game and declaring wars in-game, pretty sure people can still game the process