Honestly, the art direction for the next game could go eitherway.
On one hand, you could go back to a more realistic artstyle, similar to Melee and Brawl. Hopefully the colors won't be as muddy or desaturated. I kinda doubt they would do this, and even if they did, I'm sure not everyone would like it.
On the other hand, you could try a cel-shaded artstyle, similar to Jet Set Radio or Guilty Gear. As cool as that would be, it'd probably take up most of the resources, especially if they'd have to recreate the movesets and/or make new movesets in that artstyle, so I'm not sure they'd go for it.
Eitherway, it probably wouldn't sit well for certain characters.