Super Mario Land 2
This Gameboy classic needs some love in Super Mario Maker 2. Super Mario Land 2 would add a unique monochromatic color palette to the mix along with some new enemies and the bunny power-up.
It may be blasphemy to say that they would need to have some way to give this style color. I enjoy the original look of the game as is, but the moon mode could add some color to the world.
Even a toggle between classic and updated colors would be reasonable. The old aesthetic is endearing to some, but certainly not to all.
Going even further back, they could borrow the plane from the original Mario Land for this style. It would be an ideal stand-in since there wouldn’t be a flying, fire-breathing Koop Clown Copter in this style.
Some of the best levels in the last Super Mario Maker were shmups. The plane would be the perfect vehicle for that genre of level.
Mario Land 2 may have also been the first Mario game where the final boss could use the same power-ups as you. Nothing could be more terrifying than a stack of fireball Warios with a bunny Wario on top flying them all around.