Mandelson gave up any asperation of becoming PM when he took a seat in the Lords. It's effectively impossible to lead a Government from there now.
If Gordon Brown steps down David Milliband's probably the front runner, I'd say.
There are a number of Liberals you may remember Simon Hughes from the last contest and Vince Cable of course is currently doing himself no harm at all with the public right now.
I suspect David Davis is still skulking in the corner licking his wounds hoping to get a chance to oust Cameron