With an ulti so obvious and the damage not so high, she should be able to cancel with a marginal penalty for activating aka short timer reset, say 30-45 seconds.
She's in a good place otherwise, this would just make people actually want to play her. 
... Her ult already resets to 30 seconds if the channel is cancelled.
How would you implement it though? The R key already makes it so you can activate it early, and she can move during the channel (right word?) time.
I do know that if you let it charge for too long it cancels and goes on a shorter cooldown. Or maybe the shorter cooldown is when it's interrupted.
