How far do you think Cregan Stark could have plausibly gotten in his original quest to destroy the Great Houses that were Green supporters if he wouldn't have been talked out of it at the end of the Dance of Dragons?

[Mario Seddy]

Could Cregan Stark have succeeded in destroying the green supporting great Houses like Baratheons, Lannisters and Hightower? 

[The hairy bear]

No. He couldn't do it alone, and no one would have helped him in such endeavour. At the time, he was the only one who wanted to continue fighthing a war.

[Lord Varys]

I guess he had the strength to take on Storm's End. But that's it. And even that would only be possible if folks would have allowed his army to do this - meaning if nobody had decided to help the Baratheons to break the siege. I guess the Northmen could have even stormed the place if they had the time and opportunity to move proper siege engines into place. Cregan's men came to die in battle, so there would have been plenty of volunteers to storm impregnable Storm's End.

Oldtown he could have only taken with massive support from other houses ... and it would have only worked then if the Reach had either stood aside or helped him, too.

Casterly Rock looks like an impossibility at first glance. Even with the support of other Westermen a siege would have dragged on indefinitely. But Cregan could have had the advantage of an de facto alliance with the Ironborn. If they had blocked the Lannisters from getting supplies from the sea a siege could have been over in a couple of years.

But considering how large the Rock is and how many tunnels there might be down there which might extend far beyond the Rock proper, it is impossible to say if it is even possible to really cut the place off from the outside world.