If any character from a Wario series game would make it into a spin-off title it is undoubtedly Ashley.
None of the other characters come near Ashley, in terms of popularity and attention Nintendo is giving her. Ashley would also make the most sense, who wouldn't wanna play as a cute witch in Mario Kart?
The only downside I can find of having Ashley is having to smack other Ashley's on the track with items! Forgive me! hehe. As an Ashley fan; I have to say yes, not as an Ashley fan I still have to go with her, purely because of all the attention she has from Nintendo, it wasn't even badges.
She was in Mario Maker, Club Nintendo reward, in Smash 3ds her song was chosen over every other ware song, she now has an assist, a few versions of her song, even being the only one to revive an updated one in the form of Ashley's song Ver: 2. She even got 3ds themes, before Wario! Mii custom! Hell, she was even in a posted to promote Tank Troopers, even having customs for 3rd party games like Monster Hunter.
Even at the end of the badge Arcade, the bunny explains how he "Look's forward to the future of Ms. Ashley and her newly awoken potential drawn out by the demon!" I posted the full story here in case anybody missed out.
https://warioforums.com/index.php?threads/the-badge-arcade-story-of-ashley.2562/
So yes, I think Ashley is the most likely candidate for any Mario Spin-off, she's already in one, let's go for another!