I ask that because I assume it's not a fighting game. Of course it's not going to have the roster size of most crossover games because most are fighting/sports games.
It's not even the size, it's what they did with it.
Mario, Luigi, Yoshi, and Peach are the most uninteresting and bland choices you could pick for a game like this (as well as Toad and Bowser but they're not even playable it seems) because you absolutely expect them in ANY Mario spin-off, so to have them be the only Mario characters you can play as is just bland and boring. It's acceptable in main series games but really not appealing in spin-offs, since that's supposed to be where we can expect characters like Wario and Waluigi to shine since they don't appear in the main series titles.
That's why I think they should have had 4 more Mario characters, and have them be playable in the main story or whatever.
Then have a "Team Mario VS Team Rabbids" multiplayer mode where Team Rabbids has Rabbids versions of the Mario characters with the same abilities/stats, effectively making it similar to the "Red Team VS Blue Team" distinction in games like Team Fortress 2.
Making the Rabbids versions of the Mario characters into standard characters with possibly their own stats and abilities seems like a waste of resources in my opinion since the Rabbids aren't really "characters" in any sense of the word - they're closer to Toads and Lumas where they are these general character races that have a specific vibe and feel to them - and even that's stretching it because Toads and Lumas at least have some specific instances of actual characters (e.g. Captain Toad, Toadette, Polari, Lubba), while I'm not sure that Rabbids do (I'm admittedly not well informed on the intricacies of Rabbids lore, though).
Of course I'm already speculating far too much on a game that may not even exist so I think I'll just shut up here.