The trailer after the break finally reveals the mystery man to be Gouki all this time, who says he will repay his debt to Kazumi when the time comes. His involvement in the Mishima Saga appears to be canon, and this marks the first time ever in fighting game history that a guest character played a key role in the host game's main story.
The trailer also previews a new stage reminiscent of Gouki's Kousyu Street stage from Street Fighter III: 3rd Strike, as well as new pre-fight animations and alternate costumes for the other fighters.
Endnote: Thanks to this announcement, I'm now expecting Gouki to also be included in the console version of Tekken 7. To me, him being included in Fated Retribution seems like a prototype test for Tekken x Street Fighter, a crossover which I am still waiting to happen. As far as I'm concerned, this is a good sign for that.