Do the spirits have the ability to do that though? They way I was seeing it was the Lion Turtle was giving the power to Rava to hold for Wan and she couldn't transfer it to him unless she went through him which could possibly kill him (only person they ever tried it on). If that is the case Vatu would have to get the powers from the Lion turtles as well which isn't possible because he is imprisoned.
Good point I haven't thought about. It seems though that Rava was merely a channel that allowed Wan to bend the elements simultaneously. Wan didn't need Rava to bend the elements one at a time. All Vatu needs to do is perhaps completely possess Unalaq and then he could bend all 4 elements, and maybe even some added dark powers like those dark projections he was throwing at Wan. I just think it would be a shame if we didn't get an Anti-Avatar, and I can't think of a better candidate than Vatu to give them Avatar powers.
I am kind of confused on how people started getting bending in the first place after Wan imprisoned Vatu because the Lion Turtles said that they were no longer needed and that they would no longer allow individuals to get the elements from them. Then they left.
They may have given bending to the people who thought were worthy, and once done, they left them.
But it's clear the world went to absolute shite at that moment. All but one Lion Turtle is now dead, and they clearly hunted them down. I'm thinking that other tribes hunted them to get multiple bending abilities like Wan, and when they refused, the Lion Turtles were promptly killed. Wan is pretty much responsible for total civilization collapse, and he failed to keep the shite from hitting the fan at the end. That's why the end of it is so damn tragic.