SEGA: Tails or Eggman. No one else should even have a shot. Knuckles maybe, but only after Tails and Eggman are both in. My bet is on Tails, but Eggman using his SA2 Egg Walker would be my ideal second rep. No other series even comes close. Stuff like Monkey Ball or NiGHTS would be lucky to get trophies.
Capcom: Protoman, X, Wily, god anyone BUT Zero for Megaman. I'm honest to god sick of Zero being in everything. X would be the best option for me considering y'know, the MMX franchise is HIS and it's the other big popular Nintendo-related Megaman subseries. But with how Capcom slaps Zero into anything they can (still bitter he's in Infinite), I'd not be surprised if Zero took precedence over X again :/ If it's Street Fighter getting a second rep, Chun-Li is a definite. No way it'd be anyone other than her, unless we get a last minute Ken clone. And then M. Bison after them. As for other series, Resident Evil could definitely use a rep. Jill with her RE1 design would be the best choice, but Leon would also be alright. No Chris please, he's in a similar position to Zero for me. Not as bad, but he's getting there.
Namco: Heihachi. He's a shoe-in for the next non-port Smash imo, considering Sakurai's statements on him. Tales series has an outside shot of getting a character, but it's chump change compared to how big Tekken is for Namco. My ideal unrealistic rep would be Agumon so one game could have Mario vs. Sonic, Link vs. Cloud, AND Pikachu vs. Agumon, but Pokemon vs. Digimon was only ever a thing in the US, and not as much in Japan, so I don't think he's got that much of a chance.
Konami: Snake comes back first. Then Simon Belmont and Bomberman. Done and done.
Square: Terra and/or Black Mage and/or a Dragon Quest character. Terra would be ideal as she's got the benefit of not being a generic archetype like Black Mage, and that her game is probably the most popular and beloved of the Nintendo-based FF's (Though FF4 may provide some competition). She's also got more name recognition, considering how most probably couldn't tell you the name of any DQ protag (cue people listing them off to me). I sure can't.
Honestly though, as much as I'd love some of these characters, I really want more companies to get in on the Smash Bros. party. Gimme Crash, gimme Rayman, gimme Scorpion, gimme Ryu Hayabusa, gimme Banjo-Kazooie PLEASE. Indies I'm less keen on, but if they have to come, Shovel Knight and Shantae are the only options worth discussing, and even then I'd say it'd be best for them to come as a pair at the same time. Alone they'd stick out like a sore thumb IMO.
Wow that ended up being way longer than I thought it'd be.