Sorry to bump this without actual news, I just wanted to share this awesome pic you get from beating the last boss in Arcade mode with Mr. Karate:
Also, I may have mentioned this before, but beating Arcade mode with Billy, Raiden and Hwa unlocks this awesome art too:
Finally, I found 2 more cameos that I didn't notice before:
Dojo Stage: Akari peeking from a drawer behind the decoration swords in the middle
Isaka Stage: Shingo (or a cosplayer perhaps?) cheering from the building in the back of the right side of the stage. I think he's possibly a cosplayer because there seems to be a Kyo-look-alike cheering next to him, so they could be just fans, and they both look younger than the actual characters.
Edit: Ending arts were taken from this link:
http://www.game-art-hq.com/king-of-fighters-xiii-kof13-official-artworks-from-the-console-versions/