I think Aurelion Sol W should have no cooldown.
It's cost already mana cost on use, and is canceled by heavy CC. It's his only way to keep a melee champion away with Rylais, because a melee champion face to face with Aurelion Sol is very very Difficult to do anything about it. His Ultimate is the only skill that reposition a melee champion away from him. If you guys think it will be too powerful then increase the cost on use and lower the cost per sec.
Applies a 40% Slow.
An extended Aurelion W Will slow the enemies hit by it by 40%. If Aurelion can keep the W hitting the melee champion lets say Darius, Darius should not be able to be in melee range to kill you because he is 40% slowed. While Aurelion can keep his distance using his W. Now if Darius gets in melee range you want to reactivate your W to slow him then Push him away with R and W again to slow him again.