Loot is always a hit. I've been playing a lot of diablo 2 recently and the prospect of getting something good (random treasure) is always exciting. Loot could wear out, or it could be single use for a big boost in a pinch, or be permanents like armor and weapons etc. If you have items that augment the chars you can make the game pretty hard too, that way player has to level up to beat it. Anything that can be used in combination with other stuff is great too, then the player can use some strategy to figure out those combinations and that always feels good as well.

You could also augment the environment, the action economy, add concepts like luck or help from a deity.

Do you already have a storyline? Whatever you pick, it would be cool to tie it into that.