Well according to Zeratul and Tassadar this Great Hungerer is actually the Overmind. The Overmind was specifically created to destroy the Protoss and it was not his choice. He has no free will and must do as the Fallen One commands.
From what I gathered from the prophecy, the Xel'Naga transcended their creation, which is to say they might have evolved into some higher life form. They created the Protoss as their legacy.
This Fallen One could have been some pariah or outcast from the Xel'Naga that ended up hating the Protoss as they were something like children to the Xel'Naga.
Of course this is all speculation on my part and all of the Xel'Naga could turn out to be some arrogant godlike creatures that like to mess with the lives of what they see as "their property".
Personally Heart of the Swarm and Legacy of the Void couldn't come soon enough. Then maybe I can finally see what's all this "Apocalypse" mumbo jumbo about.
Actually, the Xel'Naga "uplifted" the Zerg for "Purity of Form" and the Protoss for their "Purity of Essence." The two species were intended to merge, and the resulting hybrids would be the form the dying Xel'Naga would take on. The Dark Voice/Fallen One (presumably a Xel'Naga, given that he's a hybrid in the game) forced the Zerg to fight the Protoss in hopes of speeding this process, and since the Zerg all but wiped out the Xel'Naga as well, I assume the Dark Voice was a malevolent Xel'Naga rebel who intended to rule as their supreme being, and reshape all life in his own (new?) image.
Yeah, Blizzard kinda threw a rather confusing curve ball at us with that. I mean here we have in WoL Kerrigan getting cured from the Zerg infestation and in Heart of the Swarm we learn it'll all be about the Queen of Blades. Maybe this Heart of the Swarm will be during the 4 years absence of the Zerg swarm, about how Kerrigan got her awesome yet deviously evil powers.
As for the Leviathan I'm thinking maybe, but then again maybe not. The Leviathan maybe a special campaign unit possible only in WoL, though as Blizzard has stated they will add new units to the games as the expansions add up. So it's anyone's guess.