Here's who I'd like to see make it into Smash Bros Switch. I'll list these by franchise name.
Super Mario series:
Waluigi - He's well overdue after being an assist trophy in Brawl and Smash Wii U and 3DS.
Captain Toad - He'd be a welcome addition since his series of games (and his appearances in Super Mario Galaxy and Super Mario 3D World) were pretty unique.
Kamek - I could see Kamek make the roster, after all he is the strongest of the Magikoopas.
Geno - A lot of people want him in so why not?
The Broodals - the bunny bosses from Super Mario Odyssey could work in Smash Bros with the right move set
Donkey Kong series
Dixie Kong - I can see her getting in thanks to the release of Donkey Kong Country Tropical Freeze. That and she's been long demanded ever since Diddy made it in Brawl
Funky Kong - same reasoning as Dixie Kong
King K Rool - another long demanded character and Smash could use more villain characters (other than Bowser and Ganondorf)
Kirby series
Marx - with Kirby Star Allies out (and the fact he's playable for the first time) I can see Marx making it in Smash Bros.
Bandana Waddle Dee - Same argument as Marx
Metroid Series
Ridley - Everybody wants him in, and I would love to see the Supreme Commander of the Space Pirates make it in Smash as a playable character (and the fact Metroid Prime 4 is coming soon could boost his chances of getting in)
Dark Samus - I could see Dark Samus make the roster as well, mostly cause of Metroid Prime 4 coming to Switch. Though this could fan the flames of another clone argument.
SA-X - again this choice would fan the flames of being a clone (even though it really is a clone of Samus in Metroid Fusion made by the X-Parasites that infected Samus on SR388)
Legend of Zelda series
Vaati - the villain from Legend of Zelda Minish Cap, he could make for an interesting character in Smash Bros
Ghirahim - again another villain, this time from Skyward Sword, he could make for an interesting character
Skull kid - I know he was an assist trophy in Smash Wii u and 3DS. He could make for an interesting character if made playable
Midna and Wolf Link - This duo from Twilight Princess would be interesting to see and it would give us another Link variant in Smash Bros with a unique move set.
Star Fox series
Wolf - fans want him back and I can see him returning to the roster. Though I hope he'll have his own final smash instead of using a Landmaster
Krystal - Now Krystal I would love to see, especially if she fights with a staff like she did in Star Fox Adventures (and with both her outfit from Adventures and Assault)
Earthbound/Mother series
Ninten - If you can have Ness and Lucas (the protagonists from Earthbound and Mother 3) then why not the first protagonist from the first game in the series?
Claus - Lucas's brother would make for an interesting character, especially if he has his alter ego The Masked Man as an alternate costume
F-Zero series
Samurai Goroh - I know he's an assist trophy in Brawl and Smash Wii U and 3DS, he could make for an interesting character if he joins the roster.
Pokémon series
Incineroar - The final evolved form of Litten could work as a playable character, with it's move set it's possible it'll be in to represent the 7th Generation of Pokémon.
Decidueye - same argument as Incineroar.
Mimikyu - Yeah, I know what your thinking but consider it's jealousy of Pikachu and the fact it could make the roster. it's Z-move could make for a brutal final smash.
Sonic series
Tails - Everybody's been pulling for Tails to make the roster, and after the Tails costume for mii fighters was added, this could be a sign Tails will make it into Smash Switch
Knuckles - same argument with Tails, not to mention the success of Sonic Mania
Shadow - he could possibly make it in, after all he was an assist trophy in the last two smash bros games
Princess Sally Acorn - Now wait, your gonna probably ask me "Why Sally? Amy Rose should be in smash bros!" and to answer, Sal's had a rough existence in the comics and her last video game appearance was a cameo in Sonic Spinball (and several fan games and rom hacks of Sonic 1 that added her as a playable character). And since Archie's Sonic comics was canceled and IDW hasn't brought in the Freedom Fighters. I think Sakurai should bring Sally back after all she's been through the past decade (not to mention one of the writers of the old archie comic was a Sonamy shipper who did the infamous shot of Sally slapping Sonic and acting psycho which was OOC for Sally to do)
Dr.Robotnik aka Dr.Eggman - Again the success of Sonic Mania could push Robotnik into Smash Bros, and besides he'd be well overdue since his last appearance in a smash game was a trophy in Brawl.
Megaman series
Protoman - I could see Protoman make it into Smash (his helmet was part of a costume for the Mii gunner)
Bass - again I could see him in Smash.
Roll - do I really need to explain this? Well they could give her attacks based on her appearences in Marvel vs Capcom or Tatsunoko vs Capcom.
Megaman X - He did cameo as part of Megaman's final smash, I could see him as a full on character which would be cool
Zero - Same as X, he would be cool as a playable character
Other games/series
Sora (Kingdom Hearts) - if Cloud could make it into Smash then why not Sora? He could have a good move set and final smash based on the KH series
Wreck-It Ralph - I could see Ralph finally get in smash thanks to the Wreck-It Ralph sequel coming soon. I could see the Nicelands apartments as his stage too.
Fix it Felix jr. - Same as Ralph, he could make it in too.
Vanellope Von Sweetz - I can see her make it into Smash Bros, her glitch/teleport would be perfect as an up+B move
King Candy/Turbo - I could see King Candy make it in (with Turbo as his alternate skin) and his final smash could be his Cybug virus form.
Crash Bandicoot - yeah this would be a huge shock if Crash ends up in Smash bros, then again the N-sane trilogy is coming out on Switch so this could make sense
Dr. Neo Cortex - same argument as Crash.
Simon Belmont (Castlevania series) - now Simon was heavily demanded for Smash Wii U and 3DS (most likely for the Captain N reference with Megaman and Pit). He could possibly make it into Smash Switch.
Alucard (Castlevania) - same reasoning with Simon Belmont
Spyro the Dragon - Now this would come out of left field if Spyro made it into smash. He could work with the right move set and final smash. Knowing Sakurai he could sneak in Spyro's Legend appearance and Skylanders appearance as alternate skins for Spyro.
Ember the Dragon - Again this would be a shot out of left field if we got Ember too, she could have her own unique move set without being a Spyro clone (and having Tara Strong voice her again would be awesome to see)
Cynder - Same argument with Spyro and Ember. Cynder would be awesome to see in Smash, especially with the right move set and final smash. Sakurai could also sneak in her Skylanders look as an alternate skin (and getting Cree Summer to reprise Cynder's voice from the Legend of Spyro series would be a huge bonus).
The Rabbids - Yeah, I know what your thinking, why the Rabbids? Well they did have a successful crossover with the Mario series and I can see the costumes for Rabbid Peach and Rabbid Luigi as alternate costumes for the Rabbids.
Sans (Undertale) - well Undertale is coming to Switch so why not add Sans to the roster?
Freddy Fazbear (FNAF) - Well Scott Cawthon did hint at FNAF on consoles so why not toss in Freddy into Smash Bros? His move set could take hints from FNAF World (mic toss and pizza summon come to mind) though his final smash could be his FNAF 1 jumpscare.
Springman and Ribbongirl (ARMS) - I could see Nintendo pulling a crossover by having Springman and Ribbongirl from ARMS show up in Smash Bros
That's my picks on whom I want to see in Smash.