I can't see Sleep being made a core mechanic, because then you'd need to have people with Sleep in every fight. However, I could see tuning Sleep and abilities like it to make easy content easier, for instance. For instance, let's say there are two trash pulls with a single, Sleep-able mob that's nasty in a group between them. Without Sleep, you just pull them individually, but with Sleep you can pull all three together and then take the threat out of the fight, speeding things up. I'd like to see the game tuned so that "pull everything you possibly can" doesn't always work anyway, so I'd be all for this sort of thing.