I'd like a genre that hasn't been dabbled in before. Something like Mario + Rabbids. Or Dance Dance Revolution: Mario Mix. Like, maybe a dedicated Mario snowboarding game in a similar vein to Snowboard Kids, or any new sports titles that hasn't been done before for Mario such as wrestling or football. An original IP being a Mario-themed rhythm game would be so great. Have a real-time strategy game like Starcraft. Hack n' Slash like the commonly suggested Super Mario Warriors, or a beat 'em up or a shoot' em up would be so much fun if Streets of Rage or Metal Slug taught us anything. I'll even welcome a Mario Diablo-clone with open arms. A Mario FPS would be so amazing. Whatever gets Mario fans playing genres that are super fun but they otherwise wouldn't be playing because they aren't Mario/Nintendo games.
What I don't want? Another cookie-cutter Mario title. Or another port for the Switch. That includes games from New Super Mario Bros., Paper Mario, and Mario & Luigi. Alphadream is dead so we won't get that coveted Partners in Time remake and somehow a nuclear explosion hasn't been set off.
And personally, Yoshi's Island should have another go but that's because I'm biased. Someone else should develop it other than Arzest. Good Feel should make another Wario Land game.