In my eyes, there are three big Mario newcomers left: Toad, Waluigi, and Paper Mario.
I think Toad “deserves” to be in the most. He wouldn’t be particularly interesting to play as, but it’s his importance that makes me want him in. He’s been in nearly every Mario game, and is an icon of the series. I think he really should’ve been added in Brawl, or at least before Rosalina in Smash 4. I think Captain Toad growing in popularity and getting his own game since Smash 4 boosts Toad’s chances in general, although I still think they’d put in regular Toad over Captain Toad.
Paper Mario would be cool too, but I’m mixed. I never beat TTYD or 64, so I don’t have a close attachment to him, but I know that those are games are fantastic and beloved. I’d put him under the other two in how much I want him, because he’d definitely be based off his Sticker Star/Color Splash self. No way he’d have partners. If they brought the series back to the TTYD style, I’d support him wholeheartedly for Smash since they’d take elements from that.
I’d love Waluigi, with his moveset representing all the Mario spin-offs, not just sports, but Party and Kart too. The only weird thing is that he’d be a Mario character while his brother is off to the side as a Wario character. He still needs his own game at some point.
If I had to choose, I’d say the order of how much I want them in would be the order I listed them. I guess I’d also say that’s the order of likeliness in my eyes.