what does the community think about adding a cooldown to lucian's passive?
The community wants it very much. This means it is probably a bad idea. The community always wants to nerf by taking the thing that makes a champion work and gutting it. Not much interesting in preserving gameplay.
He may be too strong, but just adding a cooldown to the passive is a silly fix. It's just a way of artifically limiting his burst potential while inadvertantly limiting his gameplay. If you're so worried about his level 2 burst, just make the passive or Q damage weaker in the early game. Make the second hit 25%, 37.5%, 50% at levels 1, 6, 11, or just lower early Q damage. Or, if we want to leave him a lane bully, lower his damage scaling or lower base attack speed. Any of these solve problems without screwing the design.
Riot has actually addressed this and they've said it is something they have no plans to do, because it would fundamentally alter him. He's about slinging spells together. Requiring him to place delays in between his casts screws that entire design. It not only makes him weaker, it makes him massively less fun and completely fucks the "cast spells rapidly for more E mobility" gameplay.