The only downside i could see is that all the higher levels that can acctually kill the better bosses will have the most powerful cards :/ so say a level 20 can kill Bone Elite (example) - they get a bone elite card, but this still wont compare to someone who can kill DD for example - would just take alot of balancing and testing.
Good idea though :]
Isn't it just you vs. the computer though? As opposed to being multiplayer. I can't remember how it worked though tbh.
