There are multiple choices here that I won't say I have no preference, but if I can choose multiple, I would choose more spinoffs, crossvers and new series. Basically all three except for more core Mario games. The reason for this is my interest in more series spanning from the Mario series, which I feel is the best way to go as the core series is already titles with long lifespans, so there's no need for more core games. Super Mario Odyssey and perhaps a new take in 2D Mario is great enough to last a while.
Ultimately, I chose "Introducing new series of games with new ideas" because this seems like the best compromise for what I would prefer.
Thank you for reading.