When Taker, Edge, HHH and Kane retire, it is people like ADR, Barrett and Ziggler who will need to step up and take their place.
I think ADR will go far - his mic and in-ring skills notwithstanding, his background and the appeal of his fan base will likely mean that he will continue to be around the main event scene (more like Edge rather than Swagger or Sheamus). He may only be 33 already, but I think he has some longevity in him. I look forward to him building some more legitimate feuds over the coming year, aside from Rey Mysterio and likely Mistico when he debuts. On a final point he said in an interview that he would like to remain as a heel forever and although it might not play out that way, I hope for his sake that he stays heel for as long as his gimmick does not grow stale. Because I think some fans would find a face Del Rio difficult to cheer for.
With Barrett he has the potential to be an even greater heel than ADR - he played his role in Nexus more or less to perfection. His physical presence and ability to work a crowd means that he will also be in the main event scene for a long time to come. That's not to say he will have the title runs to match ADR though. But with Bulldog failing to win the WWE title, Barrett can potentially (and arguably deserves to) create a bit of history here by becoming the first British WWE champion. The company evidently has confidence in him by launching him into feuds with both Cena and Orton in his first year. Now a good feud with Taker would go a long way to establishing more credibility along this path.
In the interim, however, I'm a little concerned about the direction in which the Corre is heading. To me it seems just like a poor man's original Nexus and Wade doesn't need the rest of the group to justify why he is a genuine contender. Moreover if management are serious about him, they should start making him a little more dominant and ruthless. Not quite to Brock Lesnar levels of course, but enough so that he can pick up some clean wins against some big names. Unless of course they wish to turn him face, which I can definitely see happening in the not too distant future. He'd make a good face, but he's an awesome heel.
Ziggler has been steadily growing more confident on the mic and his ring work has been top notch for some time now. He certainly has the ability to make a wrestling match look good regardless of the opponent he is facing. I think his main event push as of late is completely justified, especially in the context of the 'Youth Movement'. I can also see him turning face in about 12 months or so. Actually come to think of it, with Vickie Guerrero is booed louder than anyone else these days, so the transition to face won't be too difficult if he he turns against his kayfabe girlfriend.
So all in all, I see ADR having the most success but personally I would prefer it to be Barrett because as it has been mentioned above, Barrett has a better main event demeanour and his ring work will only get better with more experience (remember that ADR has both MMA and wrestling experience). It might be a long shot but when all is said and done, both Barrett and ADR could have HoF worthy careers. Ziggler will be around for another good 8-10 years but it is difficult to envisage him being as dominant as the two above.
The fact that we are talking about ADR, Barrett and Ziggler rather than people like John Morrison suggests that the WWE still have some work to do in building up babyfaces from that generation of wrestlers...although I guess Orton and Cena will continue to fill that role seeing as they are 30 and 33 respectively.