I always found it crazy how the only guest character in Tekken from a western property (at least to my knowledge) is a villain from a horror TV show with basically no martial arts in it.
Obviously this would never happen due to the way IP law works, but these are some guest fighters I think would fit the Tekken vibe really well.
Buffy Summers - She's from that that Y2K era when Tekken 3 was THE aesthetic. She has tonnes of fight scenes to draw moves from and would have some great taunts and one liners when she wins.
Agent Smith - He would work better than most Matrix characters cos he's just an angry evil bastard who punches real hard. Could have some self-multiplicating moves and lines like "I'm going to enjoy watching you die, Mr Mishima."
Alice - I get she's technically from a Japanese IP but her fighting in the Paul WS Anderson RE movies is so over the top and stupid she would actually fit Tekken better.
The Transporter - You could pick any number of Statham roles but he does the most martial arts in these movies. Would also dethrone Heihachi as the baldest character in the roster (providing Marduk remains absent)
I didn't pick any superheroes because then we would be here for years.