When it comes to past stages:
I'm pretty satisfied with the N64 stages we ended up getting. If we ever got more back though I'd like to see Saffron City and Yoshi's Island (because we don't have enough stages called that as it is right?
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From Melee I was kind of expecting the original Pokemon Stadium Big, Blue, and Rainbow Cruise to come back since they were in Brawl, and was a little sad that they didn't. I was also hoping Corneria wouldn't be exclusive to the 3DS, but that didn't happen either. I'd like all four to come back, maybe even merge Sector Z and Corneria given how similar they are.
The stage itself would be the same as Corneria, fighting atop the Great Fox and whatnot, but instead of the old Melee background they could update it to feature Corneria from Zero, and have the ship slowly rise into space to then represent Sector Z like in the old N64 stage.
And aside from those I of course really, really want Yoshi's Story and Fountain of Dreams.
Similar to the Corneria situation, I was hoping WarioWare and Green Hill Zone from Brawl would be in the Wii U version as well.
I was also hoping for New Pork City, and was surprised it wasn't there (especially with the introduction of 8-Player Smash) until we found out Lucas was gone. When he became DLC though, I expected them to add it back in along with Pirate Ship (though I'm very glad we got this one back, love the new Omega form especially), but that surprisingly didn't happen either.
I'd also like to see Frigate Orpheon return and Mario Bros. as well. Provided they reduce the knock back and spawn rate of the enemies. And of course Summit if the Ice Climbers return.
The 3DS version has tons of stages, I'd love to see get updated and ported over to the Switch version regardless of whether it's a port or a new game. Magicant, Dream Land, Prism Tower, Balloon Fight, Find Mii and Pac Maze.