According to the Hollywood Insider, Adewale Akinnuoye-Agbaje is ready to reprise his role as the mysterious (and tough) Mr. Eko, should producers come calling. Back in season 3, Adewale pretty much sealed his own fate, asking to be taken off the show because of issues going on in his personal life. The episode when Eko dies is probably one of my least favorite episodes ever (it was a poor tribute to one of my favorite characters), so hopefully if he returns they can do him some justice!
Lost, of course, is famous for seemlessly introducing new characters into the mix as well. Supposedly, they are already casting for an actor in his mid 30s to late 50s to play Lennon:
'scruffy, edgy, charismatic, and slightly stir-crazy, Lennon can be deferential when it’s called for. He’s the spokesperson/translator for the president of a foreign corporation. He’s a wily negotiator, and far more powerful than his lowly position would seem to indicate. Recurring.'
Hmmm.... very interesting. Now we just have 5 or 6 months left to wonder who this guys is....