Here's my theorycraft thread for the initial #FFXVI trailer:

Eikons serve as FF16's "summons" and have an integral role in the overarching narrative. This is the first time the series has prominently displayed so many in a single reveal trailer.

Prior to their appearance here, FF has notably utilized summons as driving forces for lore/plot as seen in FF6 (Espers) FF12 (Espers) and FF14 (Primals/Eikons) There are striking similarities that begin to compose the hybrid nature of their execution in FF16.

In FF16, Eikons are wielded by people referred to as "Dominants". Those with the ability to summon an Eikon potentially at will or within unspecified parameters. As the trailer frames it, Joshua is an essential asset of his kingdom as he is the dominant of Phoenix.

FF16's protagonist is tasked with being a "shield" for Joshua, son of the archduke. While not a prince, he still carries high rank in his kingdom. Second to that of the emperor if we follow the European model of royal hierarchy.

Within the walls of their kingdom deliberation stirs into a conflict of interest. The white haired man mentions a "pledge of swords" as it appears the kingdom the blonde woman represents has rescinded their offer. She offhandedly mentions "Battle of the Twin Realms".

"Without the blessing of the Mothercrystal, we cannot defend our realm from the spread of the Blight" is what the black haired man proclaims wearily. This brewing war sounds necessary and could explain why they seek allies to seize this crystal. Their foe is bigger than them.

Who this foe could be is purposefully made vague. The blonde woman mentions both "Dhalmeks" and the "Crusaders". Proclaiming they were driven back, not defeated outright. These factions may very well still exist and could be in direct opposition with them.

As for the blight, he mentions how it affects their "realm" which could imply that they lack their own crystal and their foe has one in another realm. (Twin Realms, reflection). Also, could this be why this woman covers her mouth when speaking to Joshua?

It is worth noting that this man bears a striking resemblance to our MC. Could he be the knightly son of a duke/count/baron? It would give weight to the inevitable tragedy that befalls their kingdom and lend credence to Joshua and MC being childhood friends who grew up together.

One subtle hint is the earring worn by them both. (Which the protagonist is still wearing when he is older) It may hold sentimental value and the possibility that Joshua died, partially fueling the means for vengeance from our MC.

We see how dominants are able to bestow their element and power to others. It is unclear if their power mimics the actual eikon's abilites without summoning. (some people think Joshua heals the mc here which can still be true as the Phoenix is known for their regenerative power)

Other potential examples of how the imbuing process works is how magic is used based on the given element. It's possible that a dominant can give it to multiple individuals but it is unclear.

Dominants and their Eikons seem to be deterrent factors of war. Assets of such high regard typically incite means for aggression. If this opposing force knew who the dominant was, especially a child, that would present the perfect opportunity to steal it from them.

Evidence of this is sprinkled throughout the trailer. The possibility of this invasion being an inside job is likely as we see the person who cuts off the soldier's head is wearing the same armor as the man previously with Joshua.

"Imperial vipers, do they really mean to invade us?"

A betrayal is afoot by someone they considered neutral or possibly an ally. They mean to take the power of the Phoenix as no other evidence supports any reasoning for this invasion. The soldiers are even huddled around him.

Before they're able to capture/kill Joshua he manages to summon Phoenix. However, this instance provokes an unprecedented event, the summoning of two flame Eikons.

Whether someone else present is able to summon Ifrit, or if Joshua summons both himself is unclear. But this reveals information regarding Eikons of other elements. (Wind, Earth, Lightning, Water) And how there is a respective dominant for each which could be held by a kingdom.

From this incident we jump forward in time. Our MC is older and apart of what appears to be a covert group seeking the dominant of Shiva. There are key implications here that connect back to the foundations sown earlier regarding Eikons and empires.

The MC swore by oath to protect Joshua no matter what, and failed in doing so. Considering there is a significant gap in events we can only assume that Ifrit's rampage left many dead (or at least the traitors/bandits), potentially including Joshua himself.

If the emperor of their kingdom survived these events and realized that the invading army seized control of the Eikon Ifrit then it has created the need to seize other Eikons as leverage. These tattoos we see on their faces may indicate branding as repentance.

"Our kind do not question orders, we follow them."

This band of covert soldiers serve their kingdom and utilize these brandings to harness the Eikon a dominant carries. We see the mc display use of multiple elements he has seized. He can still use Phoenix as well.

Whether or not Phoenix is in his possession outright is still unclear as the nature of how a dominant is chosen and if they can imbue their power to multiple people is still unclear but worth speculation nonetheless.

As for who might be controlling Shiva, one thing about Final Fantasy tales and SE in general is that they love weaving in moments of character's childhoods and juxtaposing that against their future. For now, many speculate that this girl could be the dominant.

Narratively this would give the MC a chance to reflect and mourn with someone who also knew Joshua well, and open up the possibility for a traditional party member to join amidst an alignment following the battle with Titan. (Notice that MC is never shown with Shiva powers)

In terms of how the transference of Eikon powers might work, one thing I noticed while skimming through the trailer was this peculiar marking on one of the soldiers.

These markings, coincided with the markings we see on our MC could denote the transference or "right" to use the power of an Eikon through a dominant. Considering Yoshi P (who loves FF12) and parts of his FF14 team are working on this I don't think it's mere coincidence.

These sigils could represent their respective Eikon and serve as branding. Which means the mark on the MC may be a bastardization of this methodology to allow him to control more than one element/Eikon.

The lore is a wonderful mixture of previous elements found in the titles I mentioned. A lot of interwoven elements fused together. It's exciting seeing where it may lead as more is revealed.

One final tidbit I wanted to embellish on was the beautiful way this trailer was composed. A reflection of fire in the MC's eyes, recalling his dark past, invigorating him to press on. The clashing of destructive and immortal flames. It's proving to be a brilliant tapestry.

"The Legacy Of The Crystals Has Shaped Our History For Far Too Long" A meta declaration in both it's context within #FF16 and also the vision Naoki Yoshida and CBU3 has for this new mainline entry. The possibilities are exciting and the wait will be well worth it.

Thank you all for taking the time to read through this thread. Was extremely nervous and literally spent the better half of five hours making sure I got everything "right". I missed a few things for sure but I am very proud of this overall analysis/theorycraft post.