If you gonna turn him heel don't do it with him joining Nexus. That wouldn't really make any sense what so ever. Get this feud over with Cena and Nexus however you can. But the thing is wait for the heel turn at the most expecting time. And I think the perfect way to turn Cena heel is to be revealed as the guy that took out Taker. If you turn Cena heel most likely the mature audience will cheer him on. So you need to make his heel turn to the point where no one will cheer him over the face. And the guy for that job is the Undertaker. Yes there is some fans that do not like the Undertaker but on a consistent bases Taker gets huge pops .
The motive behind Cena taking out Undertaker well I do not really have a good one. But that's where WWE writing have to come up with something big. Have Cena turn heel and milk this storyline out to where they go at it at Mania. This gives Cena and Taker a fresh new storyline for Mania. Plus Cena and Undertaker have never really fought each other on a big ppv. I think this is the perfect timing to do it especially with Orton getting more over and over by the week. I guess it all we will tell in due time but WWE really has to have a good storyline to turn him heel or it might flop just like some of the other heel turns in the past.