I really, don't know about a sequel... maybe Matt Damon. He's 39 this year... so pretty much exactly the right age for Ennis at the end of conventional BBM and I think Damon really is a great actor. In my mind he also sort of has the right look. I could also see Colin Farrell playing Ennis... he's too young in real life to be Ennis at the end of the conventional BBM narrative... but I think he could play older convincingly.
If we're talking about a re-make rather than a sequel, I'd pick a really young actor again. I don't know exactly who... but, I really like the idea, as happened in the real BBM, of using a young cast (and then having them age through the course of the film narrative with make-up, etc.). I'll have to ponder this one and come back and post later.