Yes. There's plenty of new characters they could add, a crap ton more they could do with the presentation with the more capable Switch and without the burden of a 3DS version, and the singleplayer options are STILL wanting after 4 entries.
New character options that I can think of include:
1st/2nd party: Isaac (Golden Sun), Elma (Xenoblade Chronicles X), Fiora (Xenoblade Chronicles), Inkling Boy/Girl (Splatoon), Spring Man or Ribbon Girl (ARMS), King K Rool, Waluigi, Riddley, and that's just the start of the list.
3rd party: keep Pac-Man, Mega Man, Sonic, and Cloud. Possible Additions: Crono (Chrono Trigger), Rayman, Shantae, and again, could name more but I put more thought into the 1st/2nd party ones. Crono would be a great addition though.
There's still much they could do. People who say the series shouldn't/can't go past 4 are being silly, imo.







