I've made this thread before, quite awhile ago, but there wasn't enough facts in the manga to back it up last time.
Firstly this is the order of Susano'o powerups.
Limbs such as hands, etc > Incomplete Susano'o > Perfect Susano'o > Stabilized Perfect Susano'o.
I'm pretty sure we've all been waiting for Sasuke to show something unique in his abilities, other then following in his brother's footsteps with the genjutsu, Amaterasu etc. Both Naruto and Sasuke need something unique to their characters, they are the mains of the story. Naruto still doesn't have anything unique yet, controlling a tailed beast is a feat already accomplished by Killer B, but the thread isn't about him.
I believe Sasuke's Susano'o will fly once he's acquired the Rinnegan to stabilize it. It's already been proven that you need the Rinnegan to stabilize a Susano'o, as Madara didn't have the stabilized version of his when he was fighting Hashirama. Since we've only seen a stabilized Susano'o once, we only have Madara's Susano'o to pull facts out of. Every Susano'o up until Madara's hasn't had a real, solid shape. When Madara stabilized his, the chakra focused and created a solid shape, depicting an armored warrior. I believe this is key here.
I believe all stabilizing the Susano'o does is solidify the chakra, making it stronger and revealing the Susano'o's true form.
A common misconception is that Sasuke's Susano'o isn't a Perfect Susano'o. This is wrong, Sasuke has reached the pinnacle of what he can do with his Susano'o in his current state, he just needs the Rinnegan to advance further.
Firstly, Sasuke's summoning is hawks. This already ties him down to that animal. Secondly, his Susano'o's cloak has a beak, so it's already hinted what his final form is going to be like.
Simply put, once Sasuke has acquired the Rinnegan and stabilized his Susano'o, it will solidify and take on it's true form of a hawk and will be able to fly.
OR, the beak will remained attached to his cloak and the Susano'o will become a warrior similar to Madara's except it will have wings. I think this is a bit more possible then it turning into a hawk, since Sasuke's Susano'o's final form only has the beak as apart of the cloak.
