Player stats could be interesting, as well. If you're not able to cast a defensive ability in time, you'll get pummeled. But if you reduce your cooldown period just enough, then you'll suddenly become capable of blocking the attack--which could be a game-changer.
So, improved player stats could something worth working towards, even if no new abilities are unlocked.
Another possibility could be equipment items: The abilities could be properties of the equipment you wear, so you'd have to pick and choose what you take, and what you forgo...
Alternatively, casting an ability could cost mana, depleting your mana pool. When you're out of mana, you're unable to use abilities until it has replenished sufficiently. That way, you'd have to be strategic about which abilities you use in combat. You might be able to level up your abilities, making them more powerful or more mana-efficient. Likewise, you might be able to level up your mana pool, which would allow you to do more, and thereby advance further into the game.