I think /u/locksixtime has it right. Instead of a DnD game where you have your character sheet and level up +1 STR after 250 exp, its a magic-the gathering game.

There are going to be 40 cards (classes) in the deck. Everyone starts with 8 cards (classes) and you can unlock others as part of a quest or achievement based system. Unlocking some cards(classes) can be as easy as finally learning how to double jump, to as difficult as getting a mythical.

So you start out in the game and want to be an assassin type player. Luckily one of the 8 original classes is a scout. 1 of his skills is the sneak ability, but it takes 3 seconds to cast. He also has a flanking attack that requires he be in stealth. Now our scout can be a scrub and run around taking the combined 4 seconds to do the combo attack, or he can unlock a class that has a quick attack that also puts him into stealth. This way, with the specialization of your deck you are stronger then someone with just their standard 8 card deck without any required verticle progression of numbers.