Some small additions I'd want in SSBU, some might be bigger than others, I'm not a gamedev so I wouldn't know:
Level 10 CPU
Ability to disable certain characters (for example, if you want to remove the FE characters from your roster, or your friend is getting super annoying with Duck Hunt's can, you can just ban them on a specific ruleset)
Certain hazards-off stages fixed (looking at you, Smashville)
AT/PokeBall switch
Coin Mode
Ability to choose series symbols and victory themes for Miis.
Level 10 CPU
Ability to disable certain characters (for example, if you want to remove the FE characters from your roster, or your friend is getting super annoying with Duck Hunt's can, you can just ban them on a specific ruleset)
Certain hazards-off stages fixed (looking at you, Smashville)
AT/PokeBall switch
Coin Mode
Ability to choose series symbols and victory themes for Miis.