Here's some characters I think would be fairly good, but realistic.
1. Another Mario rep... or maybe Yoshi. Here, I actually have one of two ideas - either Kamek, who sort of fulfils a "mage" archetype, or King Boo as more of a representative of Luigi's Mansion. My own pick, however, is Kamek.
2. A character from modern Pokemon, for which I'm going to suggest Gen 8 even if Gen 9 is out by then - I'm personally going to suggest Grimmsnarl as a particularly out-of-left-field pick what with it having a pretty unique design and a mix of Dark and Fairy moves, though others such as Toxtricity, Urshifu (perhaps with a true stance-change style?) and Cinderace could be solid picks too.
3. A Zelda character. Personally, I'm actually going to suggest a weird pick in the form of Ravio, who would use various items from across the Zelda series in his moveset and thus represent Zelda as a whole moreso than just Link Between Worlds. Even so, picks such as Impa, the various Champions or even Tingle are solid too - though if Ravio weren't chosen, I'd actually go for Lana from Hyrule Warriors, as I can imagine her moveset - being in a way similar to Byleth's in that she'd use all three of her weapons in the game - being the Book of Sorcery, the Deku Spear and the Summoning Gate - being quite original and interesting in comparison to other characters like her. Other picks such as Midna & Wolf Link or Skull Kid could work too, though.
4. A new "retro" rep of some sort, be they first, second or third party. Characters like Ryu Hayabusa spring to mind, but other potential picks could be Excitebike Racer, Mach Rider, or even weirder picks such as Twinbee or Dig Dug. My own pick here is Jason and SOPHIA from the Blaster Master series, due to both retro and modern appeal.
5. An older, but not retro, 1st/2nd party Nintendo character. This, up until now, seems like the least realistic to me - but something I can see being... hypothetically possible. My own suggestions here are Chibi-Robo, Saki Amamiya, Ray MKIII and Isaac, but really any character from between the N64-Wii era could work pretty well. My pick here is Isaac, simply because of fan appeal (though my personal favourite has to be Maxwell, from Scribblenauts.)
6. A literal WTF pick. I'm thinking you could go crazy here - so why not suggest the god damn Wiimote. Seriously. You could actually make a cool moveset for the Wiimote. That aside, another really weird pick could potentially be Frog from Excitebots simply for the resulting meme, though Tetromino would be the logical pick if Wiimote wasn't possible for... whatever reason. (Jokes aside, the Wiimote is my final suggestion.)
7. Our first 3rd party - and I'm going to just say "a SEGA character." As for whether this character would be in the Sonic series or not, however, I'm not really sure. If it is in the Sonic series, I'll suggest Shadow or Eggman as my most reasonable picks, with Knuckles a close third to the pair - though if not from Sonic, this really opens the floodgates in a way. My own suggestions in that case would be AiAi, Akira, Opa-Opa, Beat (personal bias) or maybe even the damn Hornet car from Daytona USA, but able to fight as in Fighters Megamix... though my final logical pick is going to be a decloned Shadow.
8. A new Capcom character. This is a genuinely difficult one for me, because I only think it's reasonable to expect that we'd get one, but it would be between a new series, a new Street Fighter rep, or a new Mega Man rep. Some ideas I have for Street Fighter characters could be M. Bison, who I think could actually be pretty interesting, or Chun-Li to play it safe - Mega Man could potentially include Bass, Zero or even a curveball pick in the form of Wily himself - and a new Capcom character as a whole could be Phoenix Wright, Dante, Monster Hunter or Jill Valentine. My "logical" pick here is M. Bison, though my "personal" pick has to be Bass...
9. Some sort of fighting game character from a new series. For this, I'd say either Fulgore from Killer Instinct, Sub-Zero from Mortal Kombat, or Sol Badguy from Guilty Gear... though as for which one, I'm not really sure (though I'd stress that Sol would have to be an ArcSys rep similar to how Terry was an SNK rep as a whole.) I think Sub-Zero would be the most reasonable pick due to his ice abilities (as mentioned in a previous post) as well as MK's importance to the fighting game genre as a whole.
10. A general 3rd party pick, such as Ezio, Spyro, or Neku. To be honest, this is just a sort of "toss it out there" pick - I can't help but feel like it's possible that this would be a JRPG rep such as Crono, which would be cool! Neku, however, is my final pick here.
11. A big, fan-favourite pick. I'd put Geno as the big pick here, but Doom Guy, Rayman, Reimu, Master Chief and Crash could be good picks too.
View attachment 334107
Whilst some of my all-time favourites aren't exactly here, and there are some... odd picks here, I think this would be a pretty solid, balanced roster of newcomers. A good balance of weird picks and logical ones works here I think - though I don't think this would be the exact announcement order. I'd say that would be:
Event 1 (Big reveal?)
1. Kamek
2. Neku
Event 2 (Probably a Direct)
3. Jason & SOPHIA
Event 3 (Event like Game Awards or somethin)
4. Ravio
5. Shadow
Event 4 (Another Direct)
6. M. Bison
Event 5 (A bigger Direct, perhaps a special one for Ultimate Deluxe?)
7. Grimmsnarl
8. Isaac
Event 6
9. Sub-Zero
Event 7
10. Wiimote
11. Geno
So... yeah! My own personal picks would differ slightly, this is just my own take on a "plausible" one.