i think ashley's actually a likely candidate for mario spinoffs from this point (i really wouldn't have said this a year ago, lol) but when does show up, it will be as DLC, not as part of the base game. Here's why:
1: she's not a big draw. obviously she has a following in Japan, but not to the point her presence in something would make or break the sales of a game, and those would be super-excited about her inclusion are the kind that would also buy the latest mario sport games anyway.
2: I really can't be arsed to find a source atm, but I remember an article where nintendo said their plan is to release one "big" franchise game a generation and maintan sales and buzzz by releasing dlc, rathher than doing multiple sequels. this goes double if the current signs about the NX platforms having a shared library, as they would no longer make "mario kart handheld" and then make "mario kart console" a few years later.
3: all the existing ashley merchandise sold well, and they're likely to do more. there's also a lot of mario maker ashley levels which shows there is interest in playing as her.
ashley as a dlc playable character would sell well and maintain post-release interest in Japan, assuming the game isn't rushed to oblivion and bomb (as MP10 did). that's how i see it anyway.