JuicieJ
SHOW ME YA MOVES!
I actually doubt he will be sealed in the mirror of twilight. I think Ghirahim(if he's not the sword) and the rest of the tribe will be sealed. That King of Darkness would have have been in Twilight princess if he was sealed, and he bears no resemblance to any Twili, who are tall and slender like Ghirahim. I don't know how he will be dealt with but I think he could relate to Ganon, there are all sorts of things they could do so as of yet I have no theory on how.
The King of Darkness was directly stated in FSA to have been sealed in the Dark Mirror (some say it was the Trident, though, the quote is slightly ambiguous), so anything else would contradict that, and Nintendo's not going to do that sort of thing with this big of a connection for the canon. The fact that he doesn't resemble the Twili also doesn't mean anything. According to what you said, that they all have to be tall and slender, which doesn't make any sense. Not all humans are tall and slender. Not all animals (of the same species) are the same size and strength. There's no reason for an exception for the Twili.