There are some theories as to whether or not Bahamut is a 'natural' Primal or perhaps an Allagan-created (or modified) one. If that is the case, I think it makes sense than Ultima looks in some ways similar to him. We'll have to wait and find out!
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Take a look at the Ultima Extreme preview from the last Live Letter: http://youtu.be/_x_To8MeloI?t=1h51m35s
Unlike the other HM and EM primals Ultima Weapon actually has a completely new model. His armor is missing (presumably destroyed by our last fight with him) revealing that he does indeed seem to have a draconic origin. He also has wings now... Judging by the pulsing red elements of his body though I'm going to go with him being some sort of hybrid of dragon and magitek.
We know from coil that the Allagans were experimenting with controlling the dragons using magitek, and at least attempted to enslave Bahamut, so I wouldn't be surprised if Ultima Weapon was another branch of their research into some kind of cybernetics. The Heart was likely the missing piece of the puzzle so to speak.
Hrm?
No Ultima Weapon had wings before, you just didn't pay attention
As I recall Ultima Weapon gains wings upon absorbing Garuda's essence during the post-Howling Eye CS and loses them upon losing her essence during the first fight.
The opening CS in the Live Letter shows Ultima Weapon with wings, but later scenes of the fight showcase it without the wings and without the red-power line type things. (In such a way it seems to correlate with the other Ultima fight, losing the primals' essence over the course of the fight and eventually going straight to a 'post-Ultima' dealio)
So super bumping this cause of Shadowbringers revelations....
Heart of Sabik is totally from the Ancients right?
It may be from Ancient Ivalice timeline wise given the fact that Ultima came from the High Seraph that descended upon the world post split. Thus it was likely made by the Ascians to be part of their plans as a means of causing calamities. Which would explain their knowledge of how it works and why Ancient Allag had trouble trying to figure it out.
Other revelations we've had since ARR in regards to OP's post:
HW confirmed that the Primal Bahamut was created by Tiamat in an effort to resurrect him, though she saw immediately that he was not actually Bahamut
Emet-Selch confirms that he more or less orchestrated the Allagan Empire
Honestly my thoughts as of now are that I want to throttle Lahabrea for bringing it up, cackling maniacally, never mentioning it again, then getting himself killed so we can't possibly ask.
It's the most evil thing he's ever done.
EDIT: Didn't see how necro the thread is but NICE Catapult! XD