The official site currently has 3 slots for boss characters (Elraine included), so we can expect two more boss characters soon. It was said in Jump Festa 2012 that one of the bosses would be from Tales of Legendia.
Aside from the video, the official site was also updated with new info about the game. Not sure if I posted about this already, but aside from the main serious scenario, story mode can also branch out into funny "Comical Scenarios". They also uploaded the Jump Festa 2012 trailer in better quality, and you can download it here.