Speculation at the moment says that there are four known candidates for the title of Lord of Hunger. All but one are from the comics which will probably, if ever, be the place where we'll learn where Nihilus came from.
Zayne Carrick: Predicted to be a destroyer of worlds...that's it. Though according to the visions of the master, he'd be more of a Sith assassin if he does fall to the dark side. In which case a more probable candidate would be Sion.
Lucien: Has that relationship (I guess you can call it that) with a Miraluka. Connection to Visas?
Squint: Prisoner of the Mandalorians as of now though that whole idea of Nihilus being a prisoner is complete fanon.
The Exile: Maybe Nihilus wasn't necessarily a person but a spirit sort of the part of the Exile that she rejected. Only problem is if (not really concerned with canon) the Exile is female, wouldn't make much sense also when Visas looks at Nihilus, she'd be all "Whaaa?!" instead of saying "Just a man".