Gush decks with 1 gush won't be gush decks anymore. During the years gush was restricted I don't think there was any direct replacement. I suppose confidant should grow in numbers as the new prominent draw engine, maybe more painful truths and night's whisper. Thirst for knowledge should see more play, gifts too. But pyrodelver decks would get a blowout, maybe disappearing (that is no tragedy for me, decks appear and disappear from time to time).
@xouman Thanks for these insights. Restriction would be major problem for me though since I can't really afford to build another vintage deck at the moment. I though about Serum Visions maybe as (poor) replacement since it is a cheap spell (can sometimes be cast under spheres and triggers pyromancer/mentor) and lets you see more cards than Gush, although you draw one less. I am not super competitive as a player (meaning that it's not the end of the world if I lose more than I win as long as I have fun) so I think I would try that solution and see if it sticks.
AK was always reasonable, although has been surpassed. Impulse is never great, but a semi-reliable T1 play, although not half as good as Jace-V on T1 if you get to untap. Opal and Top heavy builds get Thirst. None however are mana-free, but I guess that's the point of restricting the free-draw.
There's a lot of varieties of gush decks. But I think many of them would replace 1 of the gushes with a land. Many people cheat the mana base with gush a little bit knowing they can make up a land drop or use it to protect against waste/strip effects.
Outside of that I think in many decks people would take 2 ofs and 3 ofs and add another one one of them for better consistency. When you have as much draw as many blue decks use now (gush, ponder, preordain, brainstorm, ancestral, dig, treasure cruise) you can get away with a little less consistency. But if you remove some of that draw you either replace it directly with other draw or firm up the consistency you sacrificed in the first place. Part of gush's appeal is its efficiency and I'm just not sure there are other draw card that are as efficient to make them worthwhile. So fall back to more consistency.
If Gush ever got restricted it would be in a meta with more restricted blue draw cards than there ever has been. DDT, TC, and Dack did not exist the last time Gush was banned, so really what I think you would see would be a shift to more of a Blue Good Stuff deck than a "Gush" deck.
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.