Several alternatives exists for cooldown-free skills
1.Pure Mana-cost system: skills only limited by cast-speed, allowing very fast depletion of mana.
2.Cast-speed only :item increase cast speeds, no mana costs.
3.Resource/Charge cast.Allow N charges of skill(grows with level) to be accumulated(at speed of K charges/minute) and discharged as skill casts.
4.Spells cost life or unified energy(life+mana) limiting their use.
5.Fixed Cast Speed and unlimited casts:FPS-like skills.