I have no doubt that the game will be stellar, given all we've seen from Sumo gameplay-wise. I'd even say that the fanservice is much better when it comes to the tracks, music and potential in the sticker system that we've yet to see.
However, I do think that the roster needs a bit more in terms of awesome characters and I really hope that the final reveals are as great as Ryo, Bonanza Bros and Opa Opa were in the first game (I'd say those three were the coolest additions to the roster fanservice-wise).
I have a strong feeling that the following are NOT returning: Jacky & Akira, Bonanza Bros, Chu Chus, Zobio & Zobiko. Sumo stated that there will be no multi-character IPs, and I think all of these might remian exclusive to the first game.
I also have a feeling that the following will not return: Ryo Hazuki, Alex Kidd, Opa Opa (but Opa Opa will probably be flag man)
I'd hate it if Ryo Hazuki and Alex Kidd weren't in, Alex Kidd especially given how prominent he has been in the All-Stars series and his importance to SEGA's past. As for Ryo, I just have a feeling that he might also be a character that was a super special one-off for the first game. If he IS MIA in Transformed, I hope at the very least a character as cool as him is in his place. Either a Daytona car or Segata Sanshiro.