The core design of these games by PDX is to fulfill players' power fantasy, and I am quite fond of unbound growth of my might as I play for hundreds of years in the game. If you don't want to become too powerful, you can always choose to restart the game and play shorter campaigns, which is a valid playstyle and well accommodated, by the likes of Haesteinn. You always have additional challenges, like partition succession laws. For me, I quite enjoy the reward of having endured those shitty succession laws, enacted superior ones, and snowball into the greatest entity on the map.
Obviously there are a few modifiers that should have a cap, such as the life expectancy from multiple legendary shrines. But this is a matter of finetuning for the sake of balance.
I think after a player conquered a huge empire the challenge can shift from conquering the rest of the map to keeping your dynasty in power of a huge empire for as long as possible till game end date.
This way a player's power fantasy is fulfilled, and they can find new challenges to play.
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