I assume they'd follow similar rules to Marvel Vs. Capcom? IIRC, the rules there are that characters must fulfill two or more of the following;
1. Be apart of the storyline. In this case, Subspace and World of Light. That should be everyone minus the Ultimate DLC
2. Have powers and abilities not shared with canon counterparts. This should also be basically everyone because Spirits, shields, stickers, and stuff like PK Fire for Ness and Lucas.
3. Be playable. So the entire main cast.
So characters like Pikachu and Samus should be fine, no questions asked.
Characters like Steve and Joker are trickier. They do have special powers (shields, Steve's BFR, Spirits) and are playable, so that's two of three.