I have nothing against Tengu and I find his character design quite unique and interesting too, but I can understand, why Team Ninja left him out and why he'll be probably left out in any future DOA game. He was the boss of one game (and the remake), he had his moment, but now the story long moved on and the main cast has other villains to fight. Let's end it on a high note. (To me he felt out of place and kinda miserable in the DOA4 cast.)
I really don't like Tekken's attempt, of throwing in every old character including clones and deceased ones, so you get to a character roster of about 50 characters. That's just too much, it breaks immersion. Quality not quantity please.