There seems to be a world where a third Mario + Rabbids game actually became a reality. Before the many issues that started to plague developer Ubisoft Milan, including many of its developers departing, even creative mastermind Davide Soliani, it seems like there was hope of making a third chapter in the series.
This has been noted on The Game Business Show podcast, where Christopher Dring has stated and mentioned that a third game was pitched to Ubisoft and Nintendo, but seemingly neither was interested in approving it.
"I know they had pitched another title that didn't get approved. I don't know why but it just never happened, which was again, I think, tied to Nintendo."
<social>https://www.youtube.com/watch?v=zh6uz4S6NY4</social>
Granted, the actual nature of why the game was never picked up by the companies is unclear, but it does seem to suggest that the series is very dead in the water and that we probably shouldn't hold our breath on any more chapters.