re: WarioWare
Ashley has the most clear cut potential, and is by far the most popular. She's got a strong presence in Smash already (AT, Mii Costume, 4 star spirit battle, her theme is remixed twice over...) and however minimally seems to have stretched beyond the confines of her series into the greater Nintendo-sphere the most effectively due to Ko Takeuchi's expressive love for the character. Maybe most importantly, she's just a strikingly fresh character in the context of Smash. We don't have that many magic users, let alone magicians or spellcasters. Ashley is a witch, a type of character currently absent from the roster, and with the lens of being a WW character would bring with her a great deal of cartoony silly flair and quirky effects. I'm not sure how highly I'd place Ashley on my own "most wanted" agenda but she's bursting with fun opportunity if they ever felt so inclined to take it. It would definitely require quite a few liberties, but not unlike Sheik or Incineroar's "ninja" or "wrestler" structures respectively I think a simple "witch" role is an unexplored one that already lets the mind wander.
Also just to save face,
is obviously not the kind of witch I'm talking about here lol.
Mona and Jimmy T are more eccentric, they would pull out wacky props from odd jobs or make silly dance poses respectively. I really like these two and I'd gladly welcome them to the fray, but I'm unsure they provide the same amount of exciting gameplay opportunity as Ashley would relative to others who fulfill similar goals already (Game & Watch, Villager, Wii Fit Trainer etc). Meanwhile, 9-Volt is really an idea I could never get behind. Referencing a bunch of Nintendo games doesn't really add so much to Smash Bros when all those games are represented sufficiently through other characters and items. He's a character who is able to stand out in the context of his series but not so much here, so I really don't like this idea.
Then you have characters like Orbulon, or Penny, and I think these characters would be great but do struggle a bit put up against more popular faces, those who have been around longer or those with more direct inspiration. I would kill to play as either of these two, maybe even more than any of the other ones I mentioned, but for Orbulon I feel like you'd have to make nearly everything up and Penny... I unno, she just lags behind the others a bit. She doesn't have the seniority of Mona / Jimmy / Orbulon / 9-Volt nor the popularity of Ashley, although I've noticed her climbing up a bit. And she had a strong presence in Get It Together, so I unno maybe she's a little bit of a dark horse candidate among the pack. I hope her theme song makes it in at least.