Force Annie to use or lose her stun
Not having a CD or some duration limiting her up time of her stun is kind of broken. It is not hard to charge up and its use is game changing. Being able to flash R stunning 3-4 targets and melting health bars is a pretty strong combo. But being able to just sit around holding it for forever makes any attempt at an engage by the enemy extremely hard to pull off.
So, lets add a duration for how long she can hold it for. Once she gets 4 stacks she has to use the stun within 15 seconds or that stun is lost. Casting an ability (other than your ult) that does not hit any enemies or neutral targets, extends the duration for a short time. Think of this as similar to Jhin's 4th shot mechanics wise. If she has, say 2 or 3 stacks she can hold them for 10 or so seconds before they start to fall off 1 by 1 every few seconds (so you cant just hold 3 charges instead of 4).