The problem with your suggestion for me is that by the time you have your Barrett and Miz scenario play out plus the requisite followups for both of them to not make it swagger flukes then that is a long time for Sheamus to do something else. He basically takes the CM Punk fall from grace route then.
I see your point in terms of leaving Sheamus out perhaps for too long. Yet it is completely doable. There's 2 ways of achieving it. Considering the fact that Sheamus right now seems to be busy with John Morrison, who will be doing that for at least one or two Pay Per Views to go. Which means that we're around TLC / Royal Rumble by the end of this feud. That means he's been more than busy pushing someone upwards, and still looking like a strong main eventer.
And there's still the Triple H return feud to be dealt with. There's nothing saying that John Cena vs Wade Barrett won't be the definite final answer to Barrett's occupation after his Randy Orton feud dies down. And Triple H vs Sheamus still needs to be finished. Triple H vs Sheamus would then most likely carry onto another Wrestlemania / Extreme Rules match, who knows, but it will continue for a bit. After that, we've got the draft. Send Sheamus to Smackdown and you got a problem solved - Sheamus vs Undertaker / Big Show / Rey Mysterio / Edge.
Or just keep him around and launch him into the title picture again. It all requires though that Miz or Wade gets a title reign that lasts long enough. Which would be doable by giving the belt to Wade or Miz around TLC / Royal Rumble where Sheamus returns to being busy with Triple H, or still is busy with John Morrison.
Leave Punk out of it if you want. It is unfortunate but I guess a reality that he will be further midcarded.
It's sadly true. There's a lot of big stuff going on there is much bigger than Punk on RAW. Nexus is on a roll, and Punk takes a back seat to it. That's why I didn't like him being drafted in the beginning.
How do you keep momentum behind three heels needing to stay in the title picture for establishment purposes? Especially when it is not like they are going to have heel champs year round.I already commented on sheamus, if Barrett leaves the title picture after nexus runs its course then it kills much of what they did for him and if Miz only has a quick cash in and that is it then it gets him nowhere.
They don't need to be in the title picture for establishment purposes. The biggest example of this is Undertaker, who remained relevant and established fairly well after his World title reign in the early 90's.
Establishment can easily be done through a main event non-title feud. Let's say Miz is the champion after TLC, and he retains the belt all the way to Wrestlemania, there's no failure there in terms of the Swagger reign if they let him retain properly. As well as there's no problem with in the meantime having John Cena vs Wade Barrett, which establishes Wade more than enough if he continues to decimate John, or if he goes on to decimate Triple H for a little while. Which would leave John Cena vs Miz to be doable at least once or twice leading into Wrestlemania.
It might seem confusing, but there's a decent amount of available main event guys that will be able to shuffle around to the various people and establish all 3 of them, due to Punk being out for a fair while most likely, and will return to feud with Daniel Bryan most likely.