Having the option to auto cast spells as soon as they're off cooldown would be really helpful. This could be done by simply holding the key down, the server would then process it as "cast X spell as soon as it's off cooldown" thus removing the difference between players with a low ping vs high.
For example when you're Skarner/Hecarim you're constantly losing on potential DPS because it's impossible to use the spell as soon as it's off cooldown.
In the off chance in which I got it wrong and the game client allows you to send a demand to cast a spell even though it's still on cooldown, then it would still be a QoL change for the player himself ~~and my keyboard~~. Plus skype calls would be less noisy.