First, Crowd Control has nothing to do with a single enemy for the obvious namesake, Crowd. CC is more on things like Sleep a bunch of mobs so as to let the party focus on one single odd one.
Second, if you truly want a fun and reactive fight, monsters should not have a predetermined rotation of skills. Have you seen any fighting games that is considered a little bit challenge, where the PC-controlled opponent has a predetermined routine?
Third, "something" to alert, yes. There are already effects that notify you of what the mobs going to do (The gnat disappear IS the effect, there is a delay before it appear and zap you and you CAN avoid that the moment it disappear, Dodo has an obvious casting animation and so does Apkallu). For many, it's not enough. But then we have a dilemma here: It's a reactive playstyle, you extend the time for reaction to make it easy, in effect also make it less challenging? If you play action games, say, Devil May Cry 3, if enemies fight in slow-mo while you still move around at normal speed, would it still be a challenging game? And then there's latency issue.