Thiem will probably be the contemporary version of the clay court specialists of the 90s-00s: Bruguera, Moya, Costa, Coria. He might win a Slam or two, but it only (or mainly) be because Rafa and Novak are waning when he's peaking.
Kyrgios, to me, looks like he'll slot nicely in the Philippoussis category: big serve, big weapons, not the mentality. Maybe if he's lucky, he'll up it a notch and have a Krajicek or Ivanisevic type career. Best case scenario is slim, but possible: he gets his head together a bit and is an elite player for a few years, winning a few Slams and Masters.
Dimitrov has the full package, but is getting his act together at a relatively late date. It is crazy to think, but he's going to be 26 in a few months. That's the age Roger turning in 2007, his last peak season. Maybe he has a nice next 3-4 years where he wins a Slam or two and several Masters, but it is also very possible he only gets one Masters and always finds some way to miss a Slam...
Zverev seems like the closest thing to a sure thing among all players born 1988 and later. Barring injury (which is possible, given his height), it is only a matter of when. As I said, I don't think he wins a Slam this year, but could next year. Certainly no later than 2019. Now whether he wins 3-5 Slams or 6+ depends upon how the rest of his generation pans out.