There is still the issue of those other villains that have been rumored, the other 5 or 6 of them. I don't know about them personally, so if anyone knows where that information came from, please share.
I have a hard time not thinking of that guy as not having a direction connection to the Imprisoned creature, but I agree that Nintendo, considering how strict the embargoes usually are, would just release artwork of the last "big bad" of the game. Then again, no one knows. We'll find out in a few long days, which is exciting enough. From what I've heard, Nintendo is trying to close up a lot of the logical gaps in the over-arching story of LOZ, so it's certainly possible that the character they showed off could very well be the head of the Dark Interlopers, some precursor to Ganon, or really anything, but certainly, something important.