If you don't mind me asking, which series out of those you mentioned might have a legit shot for a playable newcomer?
Speaking of Sony-owned characters... given how Sony IS still a competitor of Nintendo's, do we expect them to be willing to lend Nintendo any of their characters as PLAYABLE, or will they go the Shovel Knight route of just letting them be ATs?
Yes, I know Shovel Knight is an indie, so it's not the most APT comparison, but SEGA, Capcom, and Konami already have PLAYABLE characters in addition to 3rd party ATs, so...
Well, Golden Sun is by far the most likely, due to Isaac's Ballot performance and its general popularity. To a certain extent, this and Rhythm Heaven have tended to dominate or even bottleneck a lot of the discussion on non-80s, non-current Nintendo series (I mean before RH's recent reveal for Switch, which is why I didn't list it). I do wonder how discussion of these middle-era Nintendo franchises could've gone if, say, we got Isaac in Brawl or Ult and Chorus Kids in Smash 4, but I digress with that lament.
As for the rest, I think it's like, individually they're not very likely, but collectively there could be some chance, since there's so many of them with so many different potential hooks.
However,
speaking very generally, I think "more commercial success and a clearer idea of how they would work or have a hook --> more likely." I also think things like being internally developed or having some specific connection to someone like Miyamoto who has some clout w/in the company are advantages.
As for some other specific series:
I do think that as long as we're entertaining the idea of a mounted character like Excitebiker as a kind of surprise or retro rep, a Wave Race rep stands out as another possibility for that which could be even more surprising, had actually pretty similar sales, and also has a connection to Miyamoto, who produced.
As I alluded to, Custom Robo actually may have done pretty decent on the Ballot. It was appearing on polls here and there and
actually ranked as high as Sora in the big aggregated polling spreadsheet. Now, clearly it didn't do nearly as well as Sora on the actual Ballot, as Sora seems to be a huge polling outlier, and I don't see CR as any sort of top vote-getter, but there's a good chance it got some notice. It's still unlikely to be repped by a PC because it's basically defunct without a developer or clear future, and didn't sell all too well even when it was still ongoing, but at least there's a little something there you can point to.
I think Brain Age and Big Brain Academy are good examples of series that have a pretty good case actually on paper, in terms of sales and the fact they're still active series even if their big sales breakthroughs are well in the past. It's just, does Sakurai see the hook with either Dr. Kawashima or Dr. Lobe as fighters, and are these series too much in the periphery of Nintendo's collection of ongoing series to motivate trying to find a creative way to make them work?
Style Savvy I would have included in this category until recently. Unfortunately, with Fashion Dreamer being essentially a successor to the series but not published by Nintendo, its chances seem pretty dead now, despite its previous run of pretty good sales with a presumably low budget and high casual appeal.
Lastly, I'd like to bring up Pilotwings, because it's another one that at least on paper has a pretty good case: sold pretty well and was well-regarded critically in its day, had some novel gameplay that made its mark on the industry and pushed the limits of its consoles, was produced by Miyamoto, and was even briefly revived for the 3DS. It just again has sort of similar problems to some of the others: without it being relevant and without the hook for the moveset being as immediately obvious as some others, and on top of that questions on whether Nintendo would be comfortable repping Pilotwings with one of its lesser-known OCs, I'm not sure what the path to a PC would be. I guess if the Miis were ever given a moveset actually based on their games, you could work in some aircrafts or gadgets from Pilotwings Resort as part of their recovery. But again, on paper, it can't not come to mind when we think of inactive series not repped in Smash that have merit on par with some of the series already in Smash.