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This is perhaps a very minor issue I have with the game. I tend to have a twitchy trigger finger when it comes to hotkeys, and I'll usually notice when I toggle a spell on and off, a good example being Karthus's toggle, but that has a visual effect/sound effect around the champion himself. But In some cases I won't, such as with Ashe's Ice arrows. Although the "toggled" effect is there, it's slightly hard to notice.
I would suggest making the toggled animation brighter or somehow more noticeable. It's a very minor problem, but it would be nice.