My crazy long shot that’s never coming back as is, sadly: Dredge. I think it’s such a fun and unique mechanic, and cards like Life from the Loam are excellent examples of how it can be done well.
More realistically: I’d love to see Buyback again, an old one I loved from Exodus. Capsize every turn!
Dredge is really hard to do well though, in that it makes a card playable no matter what else is written on it, and making a card too playable, then adding dredge onto it can easily make it broken enough to pick up bans in eternal formats. It's such a small design space, but I'd also love to see a few cards try to thread that needle.
Buyback is fun, Clockspinning in particular stands out as a fun card to play, even if it's not amazing. Expanding it to creatures would be a fun twist to see it come back with.
Yeah probably so difficult that even in the 0% chance it does come back the cards would be so neutered that what’s point.
A low dredge value helps though and even a few that are quirky would be cool. Like a recurring ETB effect on a cheap artifact with dredge 2 that’s heavy on the lore. Something!
We did get a dredge card fairly recently in [[Shenanigans]], the key here was a niche (but okay priced) effect + a low dredge number. Brings it more in line with the original "fair" use of dredge (re buying useful cards)