Even though I like the idea that you get to access DLC in a manner of unlockables, it just simply wouldn't work, online mode ruins this entirely. Every participant needs to share the same data, if one hasn't downloaded Crash - for instance - and one would be using him in an online match... Well, you get the idea
borderlands 2 does this,
every 'update' is just installing the dlc for people to innevitably buy,
so we get like less-than-300mb updates while the dlc's are 100kb-unlock-keys...