I too have often thought about how cool it would be to have a good Tekken spin-off game featuring Yoshimitsu.
I can imagine that a game more like what The 4th Snake described would be more realistic than what I have in mind, though.
I always thought it would be fun to have some relatively unknown, up-and-coming game development studio take on the task of making a Yoshimitsu origin story game. However, the catch would be to make the game without people knowing that it's Yoshimitsu as the main character. Like, they'd have the main character be an unknown, unnamed ninja who's a member of this secretive ninja clan.
The leader of the ninja clan has gone mad with power, killing all who oppose him with his demonic sword. Our hero ends up going through an epic struggle to defeat their evil leader, eventually becoming the leader himself. He'd have a strong female companion whose identity would also be a mystery to the player. They wouldn't know the names or faces of the two characters, so the story and character development would have to be top notch to establish that important interest in the characters.
It'd be a cool ninja action game, fighting demons and other powerful ninja characters- the higher ups in the the clan who want to stop the young hero.
In the end, he kills the evil leader and, in doing so, takes posession of the sword and becomes the new leader. It's then that he dons his trademark armor and mask, and his female companion is seen standing next to him, wearing a white fox mask.
Then someone actually calls him by his new name, Yoshimitsu.
It would imply that Yoshimitsu and Kunimitsu were friends before he became the leader, and perhaps once he took that position, she grew a little bitter and felt that she could have become the leader instead.
I dunno, just a thought I had a while ago.
It would be really hard to keep such a high-profile character a secret, so it would probably never happen... >_>