I noticed there's no thread for this yet (unless I missed it, sorry if I did) but Geoff Johns said a character would be returning in the Rebirth event. He specified it would be a single character returning, so who might it be?
I'm expecting it to be a Pre-Flashpoint version of some character, since most of the popular DC characters have already been introduced in the DCU. I'm sure some characters are dead at the moment (haven't read a New 52 book in a couple years, so I don't know who) so it could be one of them returning to life.
Overall, I think the DCU could use more than just 1 character returning to improve for fans.