1.) The WoL's brand of Dragoon is slightly different from everyone else's because they are partaking not just of Nidhogg's essence, but of Midgardsormr's.
2.) Also since the WoL isn't Ishgardian (at least not by default), they're probably not descended from one of the bloodlines cursed by Ratatoskr's murder, which could make Nidhogg's essence fight against them a little less?
3.) When we use powers with a distinctly Red hue we're drawing more directly on Nidhogg's essence, whereas Blue powers are more aspected to Midgardsormr.
Blue: Mirage Dive, Gierskogul, Battle Litany, LB3, active Blood of the Dragon
Red: Rise of the Dragon, Nastrond, Stardiver, active Life of the Dragon
Neither: All the Spear Stuff that we learned with the Lancer's Guild that's just normal ways to channel aether through a polearm without any Draconic influence, but that fits together nicely with all the leaping strikes that we pick up after exposure to the Eye of Nidhogg.
4.) This is not to say that Estinien is inferior in the Dragoon arts to the WoL. Estinien became so saturated with Nidhogg's essence that Nidhogg was able to be temporarily reborn using his flesh, and a generous portion of that essence is clearly still in there. If anything the WoL needed a boost from the Father of Dragons to catch up.
5.) Nidhogg's still in there for real. Not just a memory, not just an energy source, but a presence, an intelligence. Mostly dormant and submissive to Estinien's will, but it may grow in time. Probably not into another catastrophic Rage Monster though. I fully believe that if we came back to Etherys in two or three thousand years, there would be five living members of the First Brood once more, just one of them would also remember having been an Elezen child in Ferndale, having been a knight in service to Ishgard, having been one of the heroes who saved the world from the Endsinger, and the lived experiences would temper the new gestalt entity. I think Estinien knows this, and is at peace with it.
6.) I think the rest of the First Brood know this and are also cool with it. Nidhogg's essence is going through an unexpected phase, but the Dragons of FFXIV are weird and absurdly complicated. Azdaja and Midgardsormr are also rebuilding themselves basically from scratch after mostly dying, so Nidhogg being a current of aether inside of this mortal isn't that weird.
7.) Speaking of currents of aether, I think Ratatoskr is still in the people of Ishgard. Maybe just the tiniest sliver, maybe not enough that she could ever truly regain memories or any sense of continuity with the Ratatoskr that was, but enough that one of the Ishgardians who have been transformed into dragons by drinking dragon blood might some day re-evolve into a new Ratatoskr.
8.) I think Bahamut is irretrievable though. The Allagans were thorough as fuck, and what essence of his lingered on may have been caught up by Tiamat's summoning ritual and incorporated into the Primal. And if being absorbed by a Primal is enough to kill one of the Unsundered Ascians, I'm going to say it could erase any possibility of an ancient dragon coming back as well.
9.) If the WoL can draw strength and abilities from their connection with Midgardsormr, pacts with other dragons could be forged that don't require the dragon's eye to actually be popped out of their head first. In fact, stealing their eye is probably just a way to force such a pact from an unwilling dragon.
10.) The surviving Knights Dragoon might just figure this out, as Ishgard and Dravania establish closer ties. Rather than the Dragoons dying out, they could be reborn from a partnership where young Ishgardian knights seek out a specific Dragon to form a personal bond with.
11.) Bonds with other First Brood, or with particularly ancient and potent dragons from later generations, would also deviate from the baseline job as its manifest by the current Knights Dragoon.
A Dragoon of Hraesvelgr might manifest ice and holy light instead of searing flames, and have themes of self-sacrifice and preservation of allies. They might even be a Healer instead of a DPS job!
A Dragoon of Tiamat might have characteristics of exceptional, unwavering durability and resistance born of long penance, and be a Tank job.
A Dragoon of Vrtra might be much more teamwork-oriented, although probably taking the greater burden of risk in a fight, as a Tank job with support elements not unlike a Paladin.
A Dragoon of Azdaja would be weird as hell, and almost certainly reflect on the ages spent lost or imprisoned in the 13th with some frightening Darkness-Aspected powers.
A Dragoon of Bahamut, if one could scavenge enough of his aether to forge a bond, could just be a weird backwards way of becoming a Summoner? Probably more of a Caster than a Melee DPS in any case, Bahamut seems to have had more of a penchant for blasting the shit out of things from a distance than his siblings. Or that could just be a characteristic of the twisted Primal reflection of him.
A Dragoon of Ratatoskr could, like Vrtra, have a great deal more teamwork-oriented traits but be more focused on providing support where needed rather than being the anchor point around which the battle takes shape. I think a Ratatoskr Dragoon would be another Healer, but a "pure" healer while the Hraesvelger type would do shields with ice effects.
I can think of a dozen other powerful dragons in the setting who might grant unique gifts. Maybe an individual Dragoon could collect blessings from multiple sources, like the WoL already has?
Anyway, I have no real proof of any of this, I just like the idea that there may still be Dragoons in the future but they'd be dramatically different than the ones who were only empowered by Nidhogg's stolen eye (and possibly the lingering wisps of Ratatoskr's essence in most of their bloodlines).
