An infinite mode would be cool. They could also do a mode a bit like Slay the Spire where there's 3-4 "worlds" and this version of the game is just the first world. (Each world consisting of a boss, 3-4 mini-bosses, and maybe 20 non-boss fights).
My guess is they'd move the princess rescue at the very end of the game and insert the extra world's and bosses before that.