That's because they still have to reveal the Fates and Shadow Dragon characters outside the main ones. There's even a chance more awakening characters are in.
If they're focusing on just three games, I honestly really hope that there's more Awakening characters. If not, then it'll basically confirm that only the most popular or story relevant characters will get a chance to shine, which would make only focusing on three games even more boring. That's just depressing to think about. At this point the roster desperately feels like it needs a "whoa, what?" choice like Agitha in Hyrule Warriors.
edit: Since the game seems to take place really, really early in Awakening's story, I'd honestly want at least MOST of the original Shepherds. Maybe letting some that could be too easily redundant drop off, like Sully/Stahl being redundant because of Cain/Abel for red/green knights or Ricken/Merric as wind mages.