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If the next 2D Mario game doesn't use the NSMB engine (can we please just move away from tilesets and get a 2.5D game already), I'd like to see them take inspiration from SMW in particular as far as handling and physics are concerned.
SMW has this quick and snappy way to its momentum that makes it super-satisfying to screw about with - it's tricky to get the hang of, but once you do, the amount of potential you have to play the game in your own way is really something special. NSMB by comparison feels weird and floaty and slippery, like a vague approximation of 2D Mario rather than a translation of it. There's not nearly as much potential here.
SMW Mario but with NSMB Mario's ground-pound and wall-jump. That's my dream 2D control scheme.
I have no clue what 2D Mario's future is at this point. The closest I can think of is Mario Maker 2 by this point, which should've been announced by now. Unless of course it gets shown off in Nintendo's 2019 dump direct. Worse case scenario is they keep supporting Mario Run as the only relevant 2D Mario, without even adding new worlds and other content to support it.