I was looking at how people hate people who have an ability that goes back off cooldown when it get's a kill, Like Kat and Darius. Would creating an item that, on activate, didn't allow the ability to reset after getting a kill be gaming breaking or just what they needed?

ya it'd probably ruin champions like kat or ap trist

An item tha screws like 5 champions while doing nothing to the rest is bad design.

The best way to address problems with how a champion's kit operates would be through changing the champions kit, creating items isn't a very wise use of in-game or developer resources in that sense, and with such items your liable to create as many problems as you address.

Also, Guardian Angel. Stops champions with kill re-sets from getting re-sets.

This would make Yi sad.

AP Yi especially, but AD would also hate it.