Change my view:
Of all the major Fighting games not yet represented in Smash, Tekken would be the worst to pick mechanically.
Not represented in Smash yet are:
Tekken
Soul Calibur
Guilty Gear
BlazBlue
Killer Instinct
Mortal Kombat (pretty much banned in Japan, so it's out of the running)
Skullgirls (maybe not for good boys and girls, but never played it -- someone fill me in)
Darkstalkers (not for good boys and girls, so it's entirely out of the running)
Dead or Alive (not for good boys and girls, so it's entirely out of the running + douchey business practices make me not want a DoA rep)
not counting SNK fighters since KoF is their crossover game, and Terry reps that.
+any I missed.
Of all of these, Tekken has
- some of the slowest movement
- the most reliance on a 4-button combat system
- the lowest emphasis on air combat
- highest emphasis on footsies/boxing
- most reliance on 3-D/arena fighter mechanics
- few fireballs/projectiles
The fighting game reps so far have represented their series' mechanics to a T, and I think you'd have to take so many liberties converting a Tekken character to Smash, that you'd be doing Tekken fans a disservice by changing their character so much when the other FG reps were so faithful (this is exactly what I think Sakurai meant when he said he scrapped Heihachi in Smash 4 when he couldn't think of a way to implement his 'movement'). Even the other Namco arena fighter, Soul Calibur, has things like faster movement, wider hitboxes, and ring-outs that makes it a more natural fit for Smash.
I can't deny that Tekken has the most crossover 'oomph' of all the stated fighting games, but I'm a guy who supports characters for the sake of them being mechanically interesting/well-implemented, over for the sake of the character getting into Smash. I'd prefer a rep from KI, MK, SC, or GG to a Tekken rep, but that's just my opinion.
thank you for watching my TED talk
and none of this essay is just me justifying why I don't want Heihachi, who is probably the FG rep with the most support at this point