Well for starters, Cena is definitely retaining at Extreme Rules. Let's get that out of the way straight off.
As for who's next in line, I don't see the Cena vs Ryback feud continuing beyond ER. Also, there's talk that Mark Henry will be going over Sheamus at the ppv. With that in mind, I think we're going to see Henry vs Cena after Extreme Rules.
Given that The Rock was meant to face Cena in a rematch at ER, and that they had to find a quick fix to it all, I was expecting them to go with Henry vs Cena for ER. I was actually a bit surprised that they went for Ryback over Henry given that Henry beat Ryback at Wrestlemania. But eh, I don't really mind. I think Henry vs Cena could be a good feud (match aside), since Henry's mic work since turning heel has been nothing short of excellent.