Okay, that would be cool as well.
It being a Mario character without a Rabbid counterpart or a Rabbid counterpart without the Mario character would be weird. At least if it's Rayman there's not as much of an imbalance, as he's neither a Mario character nor a Rabbid.
To be honest I'm not really invested in either Mario or Rabbids IP wise but these would actually get me to drive out and buy the game and DLC in a heartbeat.
I dislike all of those guys but to have access a treasure trove of Mario characters and be like "lol rabbid" just feels stupid. I personally think the "new hero" is the girl who is never shown and while that is arbuably fair I still want Rayman in more.
Would probably never happen due to square but if it did I would go back in time and preorder this game