Ashley
Chance: 33%
WarioWare is still going strong, even if the reception has seen better days. Most importantly, it's still receiving plenty of representation in Smash despite there being many other sources that Wario could draw from. Wario has appeared in many, many games of all shapes and sizes before, so pigeonholing him as just a WarioWare rep would be doing him a disservice. In fact, it could be argued that WarioWare has yet to receive a "true" rep despite the success of the series. If it does get a rep, it'll almost certainly be Ashley.
Ashley is by far the fan favorite choice for new WarioWare rep, which in Sakurai's mind would likely take precedence over the seniority of in-series competition like Mona, 9-Volt and Jimmy T. Unlike Mona, who gets a different job every game, Ashley has consistent appearances as a witch. This consistency would lend much more handily to a coherent, uniform moveset, whereas Mona would likely fall into the trap of pulling references from hyperspace (then again, randomness might be a plus for WarioWare). I don't think 9-Volt is in the running at all, since all his obvious moveset would do is take away playability chances from true Retro reps while not doing anything to bring WarioWare to mind. Mr. Thang doesn't have any notable drawbacks on his own, but for him as well as any other possible WarioWare rep it really can't be understated how big the gap in popularity is.
It's remarkable how much fan love Ashley has received. In Japan she's more popular than even some pretty prominent Nintendo icons, you'd be forgiven for thinking she had her own series. Sakurai is clearly aware of this, having done several iterations of her theme as well as making her an assist trophy and mii costume in Smash 4. Even Nintendo in general seems to know just how beloved Ashley is, seeing how much she appeared in the Nintendo Badge Arcade. Her fanbase isn't always the wholesome bunch, but with how much Ashley has been promoted regardless and especially seeing how she was presented in the Nintendo Badge Arcade I doubt Nintendo thinks that's a minus.
Moveset potential is an interesting case, since the very nature of WarioWare would be difficult to translate as anything other than a walking pile of microgame references. There's no one instance where she's shown with what could amount to a full moveset. That being said, there are quite a few potential inspirations shown in cutscenes and revealed in her general character. She has her wand of course, which she could use to cast all manner of hocus pocuses (her assist trophy represents this as giving opponents status effects, which is rather unique compared to the current playable cast). She can also fire lightning outright, even without any tools. She can brew a mean potion, which could do who-knows-what. She's created monsters before, which could feature in some of her stronger attacks. She can ride a broom, which would cover recovery nicely. Perhaps one of her strongest assets would be that her wand isn't just a wand; it can transform into her flying henchman, Red. Red comes equipped with his own pitchfork, and if the demand for spear users is anything to go by people really love their thrusting weapons. Since he's his own character, he could also double as a puppet in the same vein as Luma. Even if you never look to the microgames for inspiration, there's a surprising amount to work with.
Ashley was already represented pretty thoroughly in Smash 4, but considering how negatively people reacted to non-playable representations of characters as well as how afterwards fans often brought up how Ashley should be playable in particular I doubt Sakurai would think her fans were satisfied. Her dual personalities in the East and West may cause issues with how she's represented in the game, but presumably the localizers would be equipped to smooth things over. As for whether WarioWare needs another rep in the first place, all that can be said is that she has less and less obstacles as the list of unrepresented Nintendo all-stars dwindles. It is worth asking, though, why Ashley wasn't playable in Smash 4; WarioWare hasn't had the most stellar last few years, so if he had any hang-ups before it's unlikely that they would've been cleared up since then. At the same time, it's not like Ashley has been losing relevancy in the last few years either; Her popularity transcends her series. Perhaps the slow march of time will make her all the more attractive a pick, as the undying loyalty of her base persists?
Want: 95%
The more I've talked about Ashley the more I've warmed up to her, both in terms of chance and want. Seeing another character with a W behind their name would be a lovely surprise, and the fans receiving the ultimate reward for their dedication would be quite a touching tale. A cute design and an interesting personality don't hurt either, and if you're into this sort of thing she represents niches like magic user, female, and child, the latter two having never before appeared together in Smash. I'm still concerned about her moveset so I'm hesitant to give too ringing an endorsement, but there should be enough to work with to keep the mental gymnastics limited. If Sakurai expands on her assist trophy and makes her a fighter based around status infliction, she could provide a truly unique role for players to experience.
Nominations: Ray (Custom Robo) x5