they might of been took out, or some technical problem...

or might have been especially made for the music video...

i don't think they had time to make special stuff outside the game...it wouldn't make sense: "we don't have time to include any extras, but hey, we have made special videos for adema's song!" lol

I always thought the last secret was just a little thingy to show what revision of the game you have... *Shrug*

if there is a last secret, i doubt it would be big (i.e extra character or anything), something like that would have been leaked by now, probably the ver. code. wouldnt b surprised.

If you really think about it, and considering that it was one of the sequences mostly used for MKDA's promotion, it's EXTREMELY likely that the Last Secret is indeed the Quan Chi VS Scorpion Video, since it's nowhere else to be found in the game, and it was extremely used. I would actually love that to an extra Stage, or even a poorly designed character, since I've wanted to see that video fully again for a long time (it's easy to find the beggining of the fight, up until the sword breaking and the spear throwing, but the part where Quan Chi wins is very rare).

I think the Scorpion vs Quan chi clip was just apromo clip. I don't think there's a full version.
The last secret could be more than just 1 thing. There are still so many things left unanswered, like the hallway on the right (it's there), the portal stage mystery, why the cpu keeps going into fatality stance and taunting the crap out of you when it defeats you, the stage fatalities, the clock (12:43), konga (if he's there just for fun, why announce the tie stipulation thing on the main website? That's bad PR).
And of course, the question on whether there are anymore hidden fighters (as of late, we've been getting a lot of hints that a purple (rather than a black) ninja, could be a hidden fighter.

"The secret is in the coupons". I have found the coupons, but I don't see anyting special in them. Maybe this needs more investigation...

what coupons and where did you find them

I can't remember what Koffin they were in, but they were with the MK4 vouchers (aka coupons) and Kombat Klub phone card.