Can anyone explain to me why Heihachi is an interesting choice? Other than "because he's Namco" or "because Tekken is popular."
To me, he literally seems like the most boring choice they could make, especially when put up against the 3 other Fighters Pass characters we have so far.
This is probably generalizing to the extreme, and I admit that I know pretty much nothing about Tekken, but he seem like he would be Ryu, but old and without a projectile.
Tekken's more juggle based than Street Fighter for one. And the style of Karate Heihachi uses is different enough from Ryu and Ken. If you don't want someone old, there's always Jin or Kazuya. And those guys have demon powers to make them more over the top if need be. Ideally, you'd be able to chain EWGFs with Heihachi even in this game, though I don't know if they'd go with the input system that Ryu and Ken have. Bayo has it for a move too if I'm not mistaken. So a Tekken character should have some cool strings at least. EWGFs also break projectiles in T7, so implementing that into Smash could help with their approach against zoning characters like Mega Man, R.O.B and Samus.
Heihachi for example has stuff like his Demon's Breath, Spinning Demon and Hell Axle that are mostly unique to him or the Mishima characters. The Shotos and Mishima's might have some similarities, but it'd be like saying Marth and Link are the same because they use swords.
https://www.youtube.com/watch?v=RrfQq-k1qGA
That said, I'm a Jin main, so I'd prefer him as it is, but Heihachi would likely translate in a similar manner focusing on combo strings and juggling. If you don't like him, you don't like him. But hopefully I can give you an idea of why others might.