Charles = MIB
I reckon at some early stage of Season six we’re going to see some kind of flashback to 1977ish and in that flashback we’ll see Charles Witmore die.
This then allows for all future Charles Witmore references to actually have been the MIB (Man in black from finale of Season5).
Amongst other things, this suggests that MIB could in fact the one which has been trying to get back to the island all this time. The explains the encounter at the end of season 5, where John Locke(MIB) says “you’ve no idea what I’ve gone through to get here” (<-Paraphrased)