1. Make some limit of AP say 1 milion AP minimum to be emperor - very hard to get on empty campaigns
2. Make emperor not possible to abdicate by changing home campaign: once emperor is obtained it stays on leaderboard no matter what emperor's home campaign is (so emperor can be listed on three campaigns just to make AP barrier for next candidates).
This should be vallid for one week - sometimes emperor might want to abticate and there is nothing wrong, so the cooldown time would just prevent too fast changes.
3. Do not allow to get AP on guest/travel to player campaigns - this simply allows guilds to stay active on one home campaigns and still benefit from visiting emperor farming campaigns