I don't have this game yet, but I'm still pretty hyped for it. I don't care about lack of stages, or stages being "good". Stages don't change the gameplay whatsoever, therefore I feel it doesn't matter. It could come with just the training stage, and I feel the game would still be worth the money.
Also, I would still buy the game regardless if my mains were in it or not, there's probably someone else you can find enjoyable. Especially if your mains haven't been in a new game for awhile, it's time to start searching for a new character. If they get included later on, you can always pick them up again.