I really like this idea. I only have 2 slight issues with this:
1) It might be difficult to see Sheamus as a face (even assuming that Zeke and Gabriel can successfully pull off face turns as well, which given your storyline they should be able to).
2) How can Mason Ryan's move from the 'New Nexus' to 'The Empire' be explained?
I think Ryan's move to The Empire could be explained by him tiring of being part of a losing organization in the New Nexus, and he could proclaim after another CM Punk loss, and then he could say he was foolish to believe in Punk, and that Barrett is a true success, a true leader and a survivor, and that joining someone who actually HAS succeeded and being part of the ruling regime is wiser for his own prospects than remaining on a sinking ship captained by a failure. He'll then leave Punk after Punk loses to Cena again, and offer his services to Barrett, who readily agrees to mentor him and make him a great, successful champion, which he promises for everyone under his "rule."
The idea would be to have Barrett, after forming the Empire to keep himself in control and on top, approach a still-heel Sheamus to see if he will join him. Sheamus could then express outrage at that, and also point up the fact he's still KOTR. Then Barrett could just smile at him, point out he's still the Champion, and also throw in a snide remark about how the Irish Sheamus should stay in his place (below England, that is.) Then, a furious Sheamus only doesn't attack Barrett when the rest of the Empire arrives to back Barrett up. Barrett makes another smug gesture and then leaves with his empire in tow, leaving Sheamus seething.
Later on, in a match between The Empire and the ousted Corre (Slater, Gabriel, and Zeke) vs. Barrett, Ryan, and McIntyre, The Enpire would emerge triumphant, and then engage in a post match beatdown of Barrett's former lackeys. Then Barrett takes a microphone and emphasizes how he is returning The Empire to its former days of glory, starting with the rebellious peons that should've been put down and known their place a long time ago (metaphor to both his old Corre and the former colonies.)
However, he's interrupted mid-monologue when a still furious-Sheamus comes out of the crowd and goes crazy attacking the Empire, punctuated by a Brogue Kick to Regal, but the numbers game is too much and he too is beaten down.
The following week, Barrett can cut another smug promo after emerging victorious from a title defense against Sheamus, following Empire interference, and then go on another condescending, historical-allusion laced monologue before Kofi shows up and attacks, saving Sheamus from a group beatdown. Kofi offers a handshake but a suspicious Sheamus painfully makes his way to the back, but doesn't attack him.
The week after that, Zeke and Gabriel could come very, very close to wresting tag titles from Regal and Ryan or Regal and McIntyre, but ultimately fall short after the other two Empire members interfere. From there, Zeke and Gabriel are on the receiving end of a group attack until Kofi shows up to try and help, but is beat down for his efforts. Sheamus, however, remains in the back, preparing for a Barrett title rematch later that episode. In that match, he again loses thanks to Empire interference, and this time Barrett offers him membership in the Empire again, condescendingly. Sheamus spits at Barrett, and tells him off. Barrett orders another group beatdown.
The next week, Kofi, holding the only belt (IC or US) not in Empire hands, defends against McIntyre, and though Zeke and Gabriel show up to try and fend off Empire interference, they get the worst of it, and Drew picks up a cheap win, resulting in all the belts being held by the Empire. Later on in the show, Barrett cuts another lengthy promo in the midst of a huge celebration for the triumph of his Empire, and mocks the opponents he's faced, dismissing Zeke and Gabriel as failures again. He even gets cocky to the point of allowing his Tag Champions to defend later that night, giving his former disciples one last chance before destroying them forever.
In that match, Zeke and Gabriel engage in a back and forth match, with Barrett and his other Empire member absent. However, once Zeke and Gabriel gain the upper hand, then Barrett and his other member run out to interfere. A bitter Kofi comes back to try and fight them off, but he's on the verge of getting overwhelmed.
The referee gets knocked out, and Barrett and his other Empire buddy seize the opportunity and attack Zeke and Gabriel, while Kofi tries to recover and attack Barrett. He takes out Barret's acquaintance, but then Barrett raises him up for the Wasteland and connects. Referee is still down, everyone else is down, and Barrett does a smug victory celebration in the ring for a few seconds. Then, out of the crowd, Sheamus sneaks in again and levels Wade with a Brogue Kick, and then hits the High Cross on him out of the ring. While the referee is still down, Sheamus then Brogue Kicks one of Wade' tag champions, quickly pushes Gabriel on top, and slides out of the ring. The referee counts 1-2-3.
Barrett comes to out of the ring as his Empire joins to help him up, furiously staring at the ring and having lost control of all the gold.
Kofi, Zeke, Gabriel, and Sheamus stand tall in the ring, and Sheamus gets a mike, declaring he's tired of Wade's Empire, as are the other men in the ring, and they vow to bring down Barrett's empire and take back the titles and the brand from him. If they're the ruling regime, Sheamus declares he and his accomplices are...The Rebellion!
The four of them celebrate in the ring while Barrett yells furiously at them, and the feud is on.
That's what my plan would be if I were a writer for WWE, anyway.