I could see him as Kevan Lannister who I think has been absent since Season 1, but I can't see him having a huge role in the anticipated season 4 unless they really expedite some plotlines. I recast might give them an excuse to be mysterious, "Who is this new guy?" moment for the fans when he's merely a do over.

There's also the option of it being a new non-book character or hybrid of some.

There's all sorts of Wall Jon/Bran related arcs he could slip into, a Greyjoy is possible but would seem premature too considering the event that gets them together hasn't happened yet and I really wouldn't be surprised if they clip, cut, or alter that whole arc. I've difficulty seeing him as Mace, who is described most routinely as fat.

He could be Randyll Tarly, its not really too soon for that, or even the Griffin, or someone from Essos though not likely Dany's current local.