British character actor Struan Rodger will join the cast of the Game of Thrones as "The Three-Eyed Raven" beginning in season 4. Since the three-eyed raven has actually been played by an actual raven up to this point, I think we can safely assume Rodger will be the man behind the three-eyed raven, the most mysterious being in Westeros.
Spoilers from here on out.
As folks who have finished Book 5 know, Broodraven is Ser Brynden Rivers, a bastard son of King Aegon Targaryen IV who — actually, you can look up all that junk on his page at the Wiki of Ice and Fire. Suffice it to say he was a major political power 100 years ago during the "Dunk & Egg" short stories that GRRM occasionally puts out, but now he's the last Greenseer, and the end of Bran's quest beyond the wall.
This would seem to mean that Bran's quest will be continuing at a reasonable pace with no dawdling (although it's possible he'll get a vision or two of Bloodraven before actually meeting him in person), which is cool because Bran's storyline is one of the most interesting, baffling, and probably important arcs in the whole series. It's almost certainly how the series is going to give us answers about the mysteries of the past.
As for Rodgers, well, he was in Stardust and he voiced The Face of Boe in Doctor Who. I don't know if that proves his excellence outright, but Games of Thrones' casting has been spot-on so far — now reason to assume they'd mess up now.
And in case you're curious what a guy who's well over 100 years old is doing in Westeros, or what he looks like nowadays, here's an official painting from the 2013 Game of Thrones calendar by Marc Simonetti:
[Via Warming Glow]