I think it really depends on which direction Sakurai is taking with the newcomer selection.
We don't know if Sakurai is looking at relevancy or the Ballot for characters. Could be both, for all we know.
If it's the ballot, then Rayman would have a better shot since the Omni leak and three trophies in Smash 4 lead many to vote for him in the Ballot. He does lack Japanese popularity, though.
If Sakurai is looking at relevancy, then Rabbids would be prioritized since they had a high profile exclusive on Switch. The only problem is if Mario + Rabbids came out too late for them to be considered for the base roster.
For all we know, though, neither could be playable and could be ATs.