I could see detective Pikachu being an alt costume. I feel like this game will be the game of alternative costumes. Honestly it should've been a thing a long time ago because Nintendo games have some really great costume changes. Mario Odyessy is littered with new costumes for Peach, Bowser, and, of course, Mario. Plus the game could inspire costumes for Rosalina. Heck as far as costumes for the Mario crew go we've been sitting on gold for years. Were are the Mario biker costumes for Peach and Rosa?
If we did get a detective Pikachu it would surely be a quick clone type character ala Dr. Mario (which btw is not a mainline game and therefore breaks the "rule"). I'd be willing to bet he'd be slower with a shorter recovery, but with meaty electric attacks. Maybe a charge-and-stun for his Thunder Wave.
I'm really interested to see what happens with transforming characters. A big reason why they split in the first place was because they couldnl't get it to work on the 3DS, but also it wasn't a very useful feature anyway and most of the time people just played as one or the other. I'd prefer they keep it this way honestly.
Zelda's going to be another interesting topic. She's probably her BotW self, so the OoT moveset is pretty outdated. At least for Link he still does the same general stuff, and you could just make some aesthetic changes.
I'd love to see Mario changed too. Super Jump Punch could be replaced with hat bounce or something, and maybe fludd could be replaced with Gushen (octo enemies that shoot water, so functionally the same attack.)