Wes has been superb since his return, I hope he keeps his place. He needs to be playing to bury his recent troubles.
Silvestre has also been great at CB against difficult opposition, but his attacks down the left are so vital, its hard to see him leaving LB, though its good to know he can do the biz at CB too.
O'Shea is still so young so there is time for him to try different roles, he will learn a lot to play the sorrounding positions, he will gain confidence and SAF gets to see what he can do. He certainly looks like a CB, but he has the capacity to attack with the ball too, IMO he attacks defenders better than Gaz or Wes. It is also difficult for attackers to round him, with his big frame and quick feet. Its too early to tell definately what his position will be. He does look more like a CB, he doesn't have the dynamic of a typical fullback-, though he seems to have the equipment to do well anywhere at the back(he needs plenty of competitive games and especially at fullback to explore that option) and he will only improve.
Rio has come in to be the 'bedrock' of a new defence, and I don't doubt that once he has settled and shaken off the injuries he will show his class, he looks like the natural leader of the defence, and probably the team sometime in the future.
It looks like we have three young quality CBs in Rio, Wes and JohnO, not a bad 'problem' to have. Rio-Wes looks complimentary to me, and they could continue the partnership for England, experience that could only make them stronger.
Rio-O'shea would be my second choice of two very good options. But assuming Wes stays strong, his experience gives him a slight edge as a CB, especially with O'shea looking quite capable as a fullback(watch out Gaz) .