Have you ever played Team Fortress 2 ? Something great in this game is when you want te be healed, you press a key and there's a call-out that appears upon your head. There should be something like this in LoL, but customizable. So during a fight, you could call for "STUN" or "SLOW" : it's quicker & more understandable than saying it on ventrilo/teamspeak, and it would make strategies easier to make in public games.

I also think about a "quick request menu" such as in World in Conflict or Battlefield 2. It's quite quick and it provides player with a way to request actions without having to "ask them" in the chat or with the VoiP. It really makes communication easier, especially when we don't speak the same language.

This would be fantastic.

A selection of popups for yourself, allies, or enemies.

Enemies: Target, disable, general ping.
Allies: Back, Help(me), Help(him), general ping.
Yourself: Heal, Buff, Follow(?), general ping.