What if windwall bounced skills instead of cancelling them?
It would be hilarious to see. Just imagine Syndra ulting, and all those spheres just being shot back in all directions. XD
It would also allow some very nice plays, like redirecting an enemy Blitzcrank's grab on your allied Amumu for a free initiation.
And it would also be open for spectacular misplays, like Yasuo windwalling behind himself and, as a result, being hit twice by an enemy Nami's tsunami.
EDIT: Why do so many keep getting get the idea that I wrote that "abilities change ownership"? I wrote "redirecting an ENEMY Blitzcrank's grab on your ALLIED Amumu". It means that the grab is still owned by Blitz's team, it's just redirected on the Amumu that is on Yasuo's team.
EDIT2: Targeted abilities and AAs could actually be treated like Twitch's ulti. Meaning, they can be transformed by the wall into linear skillshots (but then set to disappear after they hit their first enemy or get out of range). (Idea by deadlychuck)