The theory is pretty much blown out of the water when it comes to John Cena becoming a legitimate member of the Nexus, after everything has happened thus far. Making him lead the Nexus on the creative side of things doesn't make sense. Here, let's look at the facts:
1) Wade Barrett has belittled Cena like a slave. Why would Cena want to join the group after all the treatment he has received from them?
2) Husky Harris & Michael McGuillicutty have joined the Nexus, two men that Cena hates with an undenying passion as it was them that cost Cena the match at HIAC. He can't co-exist with them.
3) Barrett stripped Cena of the WWE Tag Team Championships after he won the match single-handedly. That was Cena's only thing that he had where he could be himself and not have to worry about Barrett's orders for once.
4) Attacks within the Nexus have occurred on both sides, with Cena eliminating the group members (i.e. Tarver) and/or the group members attacking Cena (i.e. Barrett's clothesline).
That's more than enough evidence that suggests he won't be staying in the Nexus as their leader. If they do, they'd have a lot of explaining to do... maybe about half an hour's worth of talking on why the turn happened (TNA, anyone?) But, suffice to say, we can't fully disprove a Cena turn because of how he has acted in the past and what could potentially happen. This is the only way to logical book the turn:
The match at Survivor Series pretty much indicates a lot. Cena loves the WWE and hates the Nexus... the only way to stay in the E and leave Nexus is to cost Randy Orton the championship at the PPV via officiating. Barrett becomes WWE Champion, Cena gets his release and Orton becomes quite pissed. However, after all that he has done to help the Nexus, the crowd is definitely going to be very vocal and boo Cena for what he has done... not to mention the crowd backing Orton. Considering that Cena relies on the crowd to keep him going (as it is the explanation as to why he doesn't want to get fired), he'll try and turn to them for support, which they will definitely not give. Allow a few weeks for Cena to become depressed and be in a mentally-emotional state and viola... you can turn Cena, have him berate the crowd for how they have treated him and continue being the same character he was... except in it for himself and using the same moveset that "bored" the crowd, making people loathe him even more.
He doesn't need the Nexus to become a heel nor does Barrett need to step behind Cena in the ranks... the heel turn can take place without the Nexus, that's to say if the E management definitely want a heel Cena.