He can't really be a WarioWare Character since he's never made an appearance in a single Wario game, so as weird as would look, he has to be a Mario character. He could get placed next to Wario on the character select screen though.
He's been referenced, but that's it.
It depends where DK and Yoshi go. Yoshi is probably going to be after DK or Mario, though at least sometimes Wario is after Mario. However, he's also been after Yoshi and DK(in that order). 4 is the only one to really not follow a logical pattern, but it's possible Bowser Jr. and Rosalina were late ideas or something and got oddly added in the order they're in. It does make sense in terms of "game appearances", at least. That said, basically, Yoshi was before Wario even then, not Mario, proper. DK and Yoshi have also been changed which order come first. Wario sometimes comes before or after DK too. It's not all that consistent beyond currently Wario not being directly next to Mario in order.
That, and 4 had characters split up in a less than logical order, so it doesn't mean much either. Again, the Clone Trio was in a pretty odd spot. Then the DLC came about and showed it didn't have a very consistent order anyway, so it was never a good CSS to look at for a clear consistent theme regardless. You can somewhat justify Yoshi, and even the Clone Trio. But come DLC, and the franchise order just didn't matter anymore.
We also don't know if they're going to completely change the CSS. They might remove the numbers, but still keep the same order(including Echoes only atop the originals). I wouldn't put too much stock into that kind of theory, especially when characters do often get laid out by a very clear logic. And "from the same franchise" is not necessarily the most important logic either(despite what it may seem, it keeps getting thrown out for a different kind of logic).
(To note, while the first three games did appeal to franchise order, Smash 64 barely had any characters, and was going to have exactly one character per franchise till they had enough time to make a bunch of Model Swap Characters. Then Melee was somewhat strange by not having much of a pattern(though it wasn't beyond that bad), and Brawl kept an actual consistent design. 4 completely threw it out the window by going for other logic, and Ultimate did away with it entirely. The only actual straight cases in Ultimate would be the Echoes due to being chosen from the same franchise to begin with(and if they weren't, well, they'd still be next to them), and special ones like Mii Fighters and the Transformation characters sometimes. But only cause they just happened to have the right time of reveal).