Does anyone remember when Dark Ritual rotated out of Standard or Extended? Black was terrible. Then it got some new printings and was still terrible on account of about a decade of printings that expected every black card to be powered out with Dark Ritual. In a meta where every card filter is restricted I'd expect the oops I win strategy to see more play than "fair" strategies which depend on consistency in the early turns of the game.
Edit: Blue would still be a valid colour.
I suspect that Stifle lost of a lot of its power with the printing of Misstep. Keeping Gush decks off their second land was a great strategy, though.
I think people have advocated massive unrestrictions in the past. The Tropical Storm era was actually incredibly diverse and still changing pretty rapidly when the restriction spree happened.
I find it very unlikely that we'll see unrestrictions, though. I think we're far more likely to see Gush and Shops restricted.