There are going to be "a few" new playable characters in The Binding of Isaac: Rebirth but we'll only hear about one of them pre-release. Eden is a randomly generated character that "isn't a man, nor are they a woman," according to designer Edmund McMillen. "They just are."
With Eden, we're getting a character who starts with random items (!!!), though it's a bit more complicated than that. Once unlocked, "Mom kill points" will need to be earned in order to actually play as Eden -- this is to prevent players from rerolling continuously.
You'll start with "evenly" randomized stats, one pickup item, two starting items, and one of many white hair styles. From there, good luck. McMillen says beating Rebirth with Eden will offer rewards that are "so worth the effort," but the character is one "I think everyone will love/hate using."
SPOILER ALERT! You Unlocked Eden! [Binding of Isaac]