Bear with my rambling thoughts - thinking about some of the "single rep" Nintendo series that theoretically have viable options to expand with. Every Smash game has gone through some effort to level the playing field and build up recurring series. And as I stated the other day, with the relative lack of fresh new Nintendo IPs to start plucking from, it seems likely the next roster will bulk up on some of its existing universes.
This one seems pretty clear cut to me, so it's kind of a rhetorical question. With no F-Zero or Punch-Out games in sight, and little demand for new characters, I suspect these two will remain lonely. ARMS is unlikely to expand without a second game, which we've got no murmurs of yet.
On the more positive front, Splatoon is practically guaranteed to get at least one new character in some capacity or another. Pikmin has seen impressive growth and Switch / Nintendo brand presence, likely enough to put it on the table, so there's a decent possibility here. WarioWare has also seen two Switch releases (of more dubious success?) and has a wealth of characters to choose from, with a fairly popular choice in Ashley. Probably less likely than the other two, but the series has been mobile and the support exists.
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I guess I'll use this to spring into a different question...
are there any series that feel particularly "under-repped" nowadays? This used to be one of the biggest buzzwords tossed around back in the day, but I think we've leveled things out to at least a reasonable extent for most of the longstanding series like DK or Metroid. I think the most frequent complaints that come close would be Zelda's controversial state... which is maybe more along the lines of "misrepresentation" rather than a lack thereof. And Kirby, which feels like somewhat of a similar scenario that bleeds more into nonplayable content.
Relative to their current day presence - I'd say Animal Crossing, Splatoon and Pikmin are the standouts.
Granted, this is just more of an "on paper" judgement of which series feel like they occupy more space among the Nintendo brand than they do on the Smash roster. Theoretically these series can be represented pretty efficiently, with the core mechanics and game essence being projected nicely through their existing casts. If Tom Nook were playable and Octolings were represented in some playable fashion, I think you'd at least have the bare essentials. But some of these other important series have been able to breach the bare essentials into exploring fun options outside the realm of necessity. The roster is big enough for some wiggle room and these series have enough going on to take up some extra property. Although I don't think the discussion of "under-representation" arises for these series, because it's typically reserved for classic legacy IPs and not post-2000s Nintendo. The fans aren't stirring for this the way they did for K. Rool or Ridley, or the way they're hoping for Bandana Waddle Dee now.