oh boy this is going to go well.

to help Eiyaya out a little bit lets take a look at guild wars, a game with a level cap of 20 and plenty of content to do once you've reached 20, enough that the game was able to produce several expansions and can still afford to do events every year with new items, and even expand its business with a shop.

Edit: it also has a linear main story (per expansion) much like this, though you level up while progressing along it rather than independently of it.