Thing is, most of the improvements I can think of are really just things they wanted to do before but didn't have the power to implement until now. 8 player smash was an early idea only possible until Wii U, Final Smashes was an early idea only possible until Brawl, etc.
Then there's side stuff like Smash Run that's just a recycled concept from another sakurai game, wall of collectibles that's a recycled idea from another sakurai game, difficulty options recycled from another sakurai game… and then all the bad stuff like smash tour and bosses in stages.
What I seriously wanted from Wii U was some sort of "master hand" options sort of thing where someone with the wii u gamepad could manipulate the stage, drop in items, or even just play as a master hand with the triggers and stylus. But nope, it was just smash as usual. With Switch, the main hardware advantage is it being portable with console graphics and the joy-con themselves, but I don't see how any of those would do much to add to the experience.
And then there's the stuff none of us want, like them getting ideas from others in the industry and releasing skins... in Loot Boxes or a system of the sort with double-currency economy. I mean, let's be real... if they actually had done this they COULD have made tons of money, though in the current climate it would piss a lot of people off.