Damn. It's been a while since I've re-watched the trailers...and never all that closely. So continuing my "theory" from 4 pages back, those glowing crystals in the underground look damn similar to the Timeshift Stones from SS. I know they're "luminous", but they're a totally different structure type than BotW Luminous Stone deposits, which as far as crystals go, is a more defining characteristic? (but what do I know, I'm not a geologist).
After re-watching the trailers, and a little googling about the timeshift stones, I see I'm not the only one who thought that.
And without looking up anything further, I'm probably not the only one who thinks that the mummy could be Demise. The flowing red locks, not quite as fiery as before. But that would happen if you're imprisoned underground for who knows how long. Not quite as buff as before either, as all the loose jewelry hanging off his depleted body demonstrates. And I know he was defeated and sealed within the Master Sword at the end of SS (spoiler alert). But that Demise was resurrected by Ghirahim after going back in time via, naturally, the Gate of Time.
That Demise had to be resurrected in the past because he had already been killed in the present, while in the form of The Imprisoned, by Link dropping the Sealed Grounds/Statues of the Goddess on his head (with help from the Groosenator). Or "seemingly" killed, because that type of thing can easily be retconned.
So maybe he was just almost destroyed and then kept imprisoned deep underground in his weakened state by that glowing manicured hand. And in the beginning of this trailer we see Link and Zelda stumble onto his prison, for whatever reason. So what could awaken this mummified Demise after all the time? Not sure if we see it for sure in the trailer (I can't pick out the Master Sword by the hilt, shame on me), but I am guessing that Link is carrying the Master Sword with him. The same Master Sword that has past-Demise imprisoned within it. Perhaps that's what awakens him and somehow sets him loose.
Also have a quick "theory" about how he gets loose. Cause I mean the glowing hand could maybe be enough to hold him still if there was nothing else going on. But then Zelda and Link shows up. And the mummy (sensing part of himself/his power in the Master Sword I guess) gets all fidgety, he starts twitching, his eyes start glowing, things start rumbling, flashes of light, etc...and Zelda gets a little scared.
So we are shown that the ground falls away and we also see Link reach down to save her (closeup shot of their hands - their right hands).
Then, I'm guessing, when Link is about to fall and both of them will die, the hands leaves its position on guard duty to save them (not shown, my "theory") cause of who they are.
We are shown the glowing hand reach down to grasp Link's left hand, so I am guessing that his right hand is still holding onto Zelda.
Then, I'm guessing, the red smoke attacks Link's right hand, he drops Zelda (we see a clip of her falling), then we see his arm being corrupted, we also see the glowing arm latches onto him.
And so I guess the spirit of Demise that was imprisoned inside the Master Sword escapes (we see the sword corrupted and corroded in the latest trailer), combines with the mummy, and then he floats up Hyrule Castle (maybe because it was built at the same site as the Statue of the Goddess or maybe not).
Sorry if this has been put out there by someone else, probably has. But I just had an itch to get it out there without confirming anything.