Big 52 spoilers ahead.
There are a LOT of theories out there.
Here's a small list of characters that have been talked about
Rip Hunter (Trying to hide from Skeets and other time-travelers, and using his time traveling tech to "teleport" and "send things away")
Snapper Carr (using the Star Tsar uniform)
Booster Gold (I HOPE it is, because I'd love for him to have redeemed himself after all the scandals at the beginning of 52, but the problem is that in the new issues of the Justice League, the Big 3 talk about him being dead)
All of the Linear Men (could be that they're all using the Supernova identity, but Waverider WAS just killed by Skeets)
Those are just a few. Also, here are a few comments from DC editors:
Ralph is still one of the world’s greatest detectives, so it wasn’t difficult for him to put all the pieces together and determine the identity of person behind the mask. As for you internet sleuths and would-be detectives, if you search back through all of the issues, you should be able to make a fairly educated guess regarding Supernova’s true identity. However, because it’s the Christmastime—which is the season of giving—I’ll give you all a hint: don’t look at just one person’s powers. After all, nobody said that Supernova was one individual.
And regarding Booster, DiDio has said: "The repercussions of the death will play through the remainder of 52, and although this is the conclusion of character, it’s not the conclusion of his story."