You have to "end the game" basically via various win conditions. A reign grows too strong and is capable of taking over by taking over other reigns, or in other cases you follow a princess' story line like Dragon Princess I's story which ends with her winning. Once a reign wins or a story that leads to the end is completed, the game will end and you'll be able to start a new game+ and reset everything except for I believe your level and inventory with the exception of losing all dowries (which is why you need the mimic mystery box because you can store 3 in it per box you have) but with the world reset you can do another storyline.
Also, towerfag, there was a bug with dragon princess' final story quest. After she got shot outside the tavern, human princess approached us but with no guards or anything and the game just soft locked with her there and no dialogue popping up but the game didn't crash, just stayed like that until I closed it. Next time i tried it, it was cat princess but the scene played out properly so not sure if its a thing in general or if human princess is busted for this part.
