Lots of Avatar spoilers. Nothing new (aside from predictions), but spoiling anyway.
I also assume they're going to go with some kinda of nega-Avatar, with the dark spirit infusing himself into Unalaq - but Wan didn't get his multiple-bending powers from the spirit, he still got them from the Lion Turtles.
If I recall correctly, by Aang's time, there's only one Lion Turtle left - one that grants the power to "Energy Bend" (which Aang used to take away Ozai's bending ability... which sounds a lot like how we supposedly saw Water Bending used in Season 1).
What if Vaatu originally teamed up with Amon, sought out the only remaining Lion Turtle, learned Energy Bending, and planned to use Amon to fight the Avatar... but Amon was too weak and focused on his own goals, so Vaatu chose another host - one closer to the Avatar.
Now, with this Energy Bending ability, the whole time that Amon was "taking" bending, he was actually /taking/ it, and whomever is bonded with the spirit of Vaatu now has the power to bend all five elements - just like the Avatar.