There are two things that are revealed to be planned for this game, but it ended up being scrapped.
1) Online co-op: It was planned at one time but because the developers wanted to take advantage of Nintendo Switch's standard feature of being able to play anywhere with another person, they focused on that aspect, leaving online co-op to be untouched because that would require even more resources to properly develop.
2) Backfire mechanic: Basically weapons could have a chance of inflicting a backfiring mechanic where the user will be inflicted with an effect, such as Bounce (basically being pushed back by a considerable distance) because they saw the strategic potential for such weapons, but the developers ultimately decided that such a mechanic is detrimental for the game.
If you want to know more of them in detail, then here's the link: https://nintendosoup.com/heres-why-mario-rabbids-kingdom-battle-lacks-online-multiplayer/
Thank you for reading.