Going to add my voice to the idea that the Exile zone probably isn't the best to play around with.

Global enchantments could be fun, though creatures that become them upon death could cause memory issues. Maybe bring back the flip (I think it was) cards from Kamigawa? More workable in a standard set than the Transform cards even if they have their own set of issues.

Though Library shenanigans could also be fun. Let's us through in Landcycling, Dream/nightmarecycling, and things similar to that, and ultimately I would say discard/mill ought to be a big part of the set since those are things nightmare effects usually do in MtG.

Of the suggested mechanics I have to say I like the "reveal from your hand" and sanity ones the best (though sanity should be 2 or 3 cards not 4).