Cabbagehead
#Ashley4Smashley
We already have Nikki: Nikki Brawler, Nikki Gunner, and Nikki Swordfighter!Can't say I care for Isabelle either although I see a lot of people sleeping on her popularity. However, even though she is a bonafide A-lister, her moveset potential is lackluster. Ashley, on the other hand, has crazy potential; having Red transform for attacks, inflicting de-buffs potentially, having a unique visual quirk with her hair turning white, she really writes herself.
Although I will say, I hope we don't get Nikki as a result of the former two getting in. I really do feel like that would be a waste of a slot (though I think it's more possible than people might think).
As far as Isabelle goes, you are exactly correct. She is a first-party mascot, at this point. However, she also doesn't really have anything at her disposal that hasn't already been utilized by Villager, and moveset potential is as important to Sakurai as character significance.
Ashley is in sort of a reverse situation. She more than likely isn't as popular as Isabelle worldwide (though, in Japan, Ashley is definitely sitting at the cool kid's table), but if Sakurai entertains the idea of her being in Smash, then he shouldn't have any issue visualizing an interesting moveset.
I am predicting that both will be playable in Ultimate, with Ashley being a WarioWare-ish take on the "Saturday morning cartoon"-style witch, and Isabelle being an Echo/semi-clone of Villager. Both are too iconic to simply overlook.