Never made a prediction roster, so eh let's do it. Why not?
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Newcomers:
Dixie Kong - recognizable, relevant, easy to base a model off of (Diddy's model), decently popular
Sylux - best shot at a Metroid villain that is hinted to appear in the next game whilst appearing in recent titles.
Bandana Dee - HAL using him more and more
Alolan Pokemon Trainer - don't think Sakurai has abandoned this idea entirely and won't gimp Charizard by sticking him with other Pokemon again. Hence, the Alolan Pokemon Trainer makes her mark!
Isabelle - highly marketable and promoted, popular and in a big series.
Rex & Pyra - Monolith work closely with Sakurai, move set potential is there to be used.
Inkling - ...
Ice Climbers - Sakurai loves them too much to exclude them due a previous system's limitations.
Takamaru - retro that got some attention recently. I see him making the jump from AT to fighter.
Rayman - trophy in Smash to me means something, and Ubisoft and Nintendo have a good working relationship right now.
DLC:
Greninja - think he'll just miss the initial cut due to being a lower priority, but like Mewtwo will return as DLC.
Wolf - high demand, doubt he'll make the initial cut due to being very low priority.
Crash - demand will skyrocket with latest remaster on Switch. I think he stands a very good chance as DLC.
King K. Rool - by popular demand, doubt he'll make base roster somehow.
New FE lord - none on the initial roster, but the FE Switch one will make the jump as DLC down the road.
Spring Man - still think DLC is slightly more likely than base roster for now.