Here's what I've got in mind...
Konami: Since Hehachi's limited to only a cameo appearance in one of PAC-Man's taunts, probably Bill Rizer or Jin Kazama.
SEGA: Much like with what everyone else has in mind, Arle or a second Sonic rep, though an oddball like Phantom R couldn't hurt.
Capcom: Pretty much anyone they have, though my personal picks are Sakura, Mega Man Zero, and Lilith Aensland (the already censored version of Morrigan, and I recapped on why Lilith would fit in Smash
here).
Bandai Namco: What Technomage said, but with Sora excluded. It's not that I hate Sora, but I - just in my opinion - think there's a high chance Sora won't make it as DLC.
Microsoft: Master Chief easily. I watched my dad play Halo Reach on the Xbox 360 when I was a youngster, so I have quite the history with Halo games.
SNK: Since we've already got Terry, who's pretty much the face of both SNK and Fatal Fury, I'd say we could probably fit Nakoruru and her dark side - Rera - as a Pyra and Mythra type of character. Sure, Nakoruru's a Mii Costume and a spirit, but putting her in KOF 14 put SNK out of bankruptcy. Other possible picks would include Rock Howard, who was pretty much Capcom fans' introduction to the world of SNK during 2001, and do I even NEED to explain the ultimate duo that is K' and Kula Diamond? These were probably the biggest and - no pun intended -
coolest SNK characters at the start of the 21st century, and they have been in literally ZERO CROSSOVERS with other franchises.