Here are some scattered thoughts organized by series. I'll just say up front that a lot of it is just stuff I'd like to see that seems within the realm of possibility, but with comments on said plausibility. This is also for the base game.
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There will almost certainly be a Mario newcomer or maybe two, especially since there are several good options. I think of Waluigi and Toad as the frontrunners. Others like King Boo and Pauline also seem possible.
Dixie Kong is pretty likely as a character a lot of people view as being missing.
I hope we get some kind of original, non-Triforce Zelda rep as it's long overdue. I hope the most for Skull Kid or Impa. A dedicated BotW/TotK rep makes sense, too, but idk who they'd pick. If they keep Link in BotW style and/or update Ganondorf to TotK style then maybe that makes more room for an older or recurring Zelda rep, idk.
For Metroid, Raven Beak is possible given Dread's success but seems unlikely without a planned future, unfortunately. At this point it seems likely that anything from MP4 would be too new, unless the next Smash is farther out than expected and MP4 is closer than expected.
BWD could fill a spot for a fan pick. But I'd also like to at least mention the possibility that we could get another Kirby rep, especially since Sakurai may not view BWD as important as the fans do.
I'd like to see Krystal from Star Fox but it seems pretty unlikely right now unless there would be a new game. Too bad they weren't able to get her into Brawl. She doesn't seem to get as much mention as other "previously considered" characters, which is a shame.
Pokemon...yeah I greatly dislike the narrative of resigning ourselves to an obligatory newgen starter mon. I mean, I'm not going to advocate delusion as I know the precedents for our Pokemon reps, but there are so many great evergreen options here that I have to mention them in any Pokemon newcomer discussion.
With FE I actually think they could skip out on Alear, with Engage being later in the Switch cycle, a lesser-selling, and overall more low-key release compared to Three Houses. But again I shouldn't underestimate their shilling tendencies. I will carry on with hoping for Lyn, regardless of likelihood.
Animal Crossing newcomers are definitely undertalked. Tom Nook is still kicking and has way more marketing usage than people are realizing. Like, he was all over Nintendo Live, features prominently on official websites, and has tons of official merch. Reminder that not joining until 4 is the biggest reason this series doesn't have more reps. I wouldn't even completely rule out getting two reps from here with how huge it is; KK Slider has a ton of merch while Mr. Resetti could be kind of a jokier/surprising but still very recognizable pick.
I talked about Pikmin before; basically it could get a second rep but it'll probably not be a huge priority, and would have more chance with a large number of newcomers.
I hope Xenolade doesn't turn into another "obligatory new face rep" kind of series. But we can't rule out a Xeno3 rep. Tbh I'd be fine with skipping Xeno3 despite quite enjoying this series, especially since both Shulk and Pythra had Switch appearances anyway.
Splatoon is also not to be ignored and one of the more likely series to get a newcomer. The main candidates I think are Octoling and DJ Octavio. I considered Squid Sisters as a more off-the-wall choice, but I've been leaning away from that idea recently.
RIP Spring Man
Other existing series I didn't mention I don't really see getting a newcomer (hopefully I didn't forget/overlook something).
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On to new series. Again a very undertalked aspect of speculation at this point, since many people seem to think that all the first party series we should have are here, which I think is false.
Isaac of Golden Sun is perhaps the most obvious mention here as potentially filling the next fan pick spot.
Then of course we have to bring up Ring Fit, which did huge numbers on Switch. No, not all exergames are the same; the only way Wii Fit Trainer impacts Ring Fit, maybe, is that people probably wouldn't be as surprised to see a Ring Fit rep now that we know non-combat genres like exergames are squarely on the table. In any case Ring Fit is a strong candidate series.
Rhythm Heaven deserves a mention for being most likely considered before, having some fan demand for a rep, and having a decent amount of auxiliary content in Ult. But right now there are questions about the future of the series, which doesn't help its chances unfortunately.
We don't know if there will be a dedicated "NES retro" spot again, but if there is, my pick is Excitebiker and I think they are definitely undertalked for their significance back in the day. Such a character also fits as a "surprise" rep should we get a slot like that.
I must mention Advance Wars mainly out of bias. I don't think the 1+2 remake alone will be enough, but I think the series could bring some really awesome stuff to the table and could have an outside shot if there are plans for WayForward (or someone else) to continue the series beyond the remake.
Besides these, there are a lot of other series that I think could fit either a dedicated "retro" or "surprise" slot. Wave Race would be awesome. Dr. Kawashima and Dr. Lobe also had Switch-relevant releases and have done good sales so they could be undertalked, potentially.
I think Astral Chain's chances are overrated by the fanbase but it's possible, too. And someone will def point it out if I don't mention it.
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Oh yeah and there will probably be like 1-2 third party newcomers.
I imagine some of these takes will be controversial or may even be considered totally off-the-rocker, but I welcome discussion.
