I think Letang's days in Pittsburgh are numbered and they will do everything they can to keep Malkin. We don't have the forward prospects to help replace Malkin, but we do have the defenseman prospects to help replace Letang. So if they need to clear cap space to hold onto someone or to bring someone else in, I think it is pretty clear that Letang would the guy to move.
I don't know if there is a rule against this, but I wonder if they can buyout Martin and then bring him back for less money? He might be offended that they wanted to buy him out, but I also can't imagine any other team giving him $5M/year when the cap is at $64M. I wouldn't mind bringing back Gonchar either though. He's old, but he was never a physical player so it's not like his body has been beaten down over the years.
One thing that does worry me about Letang though is how physical he plays. It is great for right now, but he's not very big and it has already started ot take a toll on him. He might not be able to remain a 60-point guy if he continues to play the way he does.