Will Sivir be viable, and what is her niche going to be?

Adcs's are really well balanced except for Corki who is too strong and Urgot who is way too weak, but those are the exceptions. Sivir is currently a little on the weak side, and since the concept of being able to play Xena in league of legends holds great appeal to me I am very much hoping that she will be viable when compared to other adcs like Varus, Caitlyn, and Draven.

The visual rework looks great, but the game play side of the rework confuses me. Reds keep saying that she is going to be a niche pick which is ok Ashe and Kogmaw are niche picks too and they are still good, but what I don't understand is what her niche is?

Is she really hard to trade with like Draven? She obviously doesn't zone people like Caitlyn with a 500 aa range. She doesn't have a lot of cc so she isn't an Ashe. She isn't gonna dash around like Ezreal and Corki.

Being a niche pick is great, but what is her niche and how is it valuable to a team?