I threw a bunch of thoughts out about FAKKU Verse some time ago. But a lot of that is pretty demanding to implement. So I'll try to come up with some simpler ideas.
Add internet points that don't mean anything. It could just be your level. Integrate it with user profiles if possible.
Add some puzzles into the game. You could probably accomplish this without adding any new assets and still make an interesting puzzle.
Add a mini-game of some sort. Something like.... play as the hen-tie and dodge the tentacles.
Hold a contest occasionally for some free faps. Or swag. To enter, users have to get a specific achievement.
Also hide some teasers and interesting trivia in the game. That can't hurt.
I've got max level gear now, and things don't worry me. I will probably keep levelling until the skeleton king bows down before me, and I don't need the furry suit