Anti-Mainstream
There are a number of ways to go about this, and one of the biggest problems is deciding what counts as mainstream. Mainstream would involve categories and mods/variations of it.
One way is to ban all cards that form a mainstream deck. Another is to only ban them from being in the same deck as each other. (Actually, I like the second one better, and I'll add to it)
You may only use mainstream cards, but not so that they make a mainstream combo/deck.