Yoshimitsu would be great. I know a Mishima is the most likely Tekken rep but my main is Alisa Boskonovitch and I'd LOVE her in Smash.
Yoshimitsu has a chance because he represents Namco fighting games as a whole. Otherwise, give us Devil Jin or GTFO. Noone else from Tekken really represents the series in the same way with the same exaggerated moves. I do enjoy the rocket powered chainsaw girl but she really doesn't say "Tekken" to me in any way.
I've also grown fond of the idea of Agumon. And of course, my girl KOS-MOS since I love the Xenosaga series.
Agumon appeals to me because I love Digimon and because he's not a Namco rep, he's a Bandai one. The other half of Namco Bandai. I'd prefer Patamon or Guilmon but there's as little chance of us getting them as Alisa and Agumon would still be a tonne of fun.
KOS-MOS, though, I don't see making it outside of Namco's own crossovers and cameos. She just isn't as well known.
Would look forward to Klonoa and a Digimon character as playables in smash.
Klonoa has the look and feel of a Nintendo character and would probably do better on Nintendo hardware, if Namco wanted to push him. Given his relative obscurity and their past treatment of the franchise, I don't see it happening.
Prince of all Cosmos from Katamari Damacy, basically my most wanted newcomer xD
An out there pick but a fun one and probably the closest thing to Billy Hatcher with a legitimate chance. I don't think he'd be anyone's first choice for a Namco rep but Sakurai doesn't seem to look at companies when picking characters. He'd be pretty left field but I could see him.
My choice, though? Nightmare. The physical manifestation of all that is evil in the Soul Calibur universe, wearing a twisted form of its main protagonist and representing the struggle between good and evil at the series' core. He is another swordsman but his focus on long move durations with high motion and reach, rather than slow startup times with big damage, would make him a very unique heavy. Especially if coupled with super armour so that opponents could get multiple small hits in per punish to rack up damage.
He's not crazy likely, given that the series has always been Namco's secondary fighter but, with Tekken previously written off, maybe he'll get his time to shine.