Either universal GCD (wow approach) or spell casting bars (GW2 approach). GW2 approach is more annoying. GCD is mostly used as an animation frame and to pace stuff right. If you have a character shooting everything in instant and then let it cool off for cd time - you get a bumpy road. Like in GW2 - try an engineer and equip rifle turret. You'll get a rifle shot at your toolbelt to use. That is one of abilities in GW2 I found that don't have any cast time tied to it. Imagine entire game built around this kind of ability - click, poof, cooldown. 5 Abilities - click, click, click, click, click (could bind everything on everything, and just smash keyboard with a fist each time anything is ready), wait for something to come off cd while watching same auto-attack animation playing over and over again. GCD in that mix forces a rotation, because you can't spit everything at once. Rotation adds tention and are easier to balance.