The thing about "Enigma's" outfit is that it looks much closer to outfits associated with the Garlean Republic (e.g. the Signifier or Medicus set) than anything we've seen out of the Allagan Empire, but it could just be a similarity in design. There are elements in the design, such as the hood and the rod, which do appear to be Allagan, and given how long the Allagan Empire lasted, it could have been characteristic of what was worn in earlier time periods. Could even be civilian apparel that we simply haven't seen yet. If so, then maybe Solus's outfit's design draws on Allagan designs, given his own fascination for the Allagan Empire.
Given all that we know about the Allagan Empire, with the Ascians teaching them how to bind Primals, and the fact that the Heart of Sabik, powering Ultima Weapon, contained a sliver of Zodiark's essence, their ties to it are strong. We don't really know which Ascian(s) oversaw it, but there is every possibility it was Emet-Selch, especially as one of the Overlords tied to the Source. Although the Allagans were an ingenuous bunch, whenever they ran dry with ideas, the Ascians very helpfully stepped in, and I wonder if this also applied to the Voidgate itself, since they are intimately familiar with travelling shards and the Void itself, electing to then impart their secrets to the Allagan magi. Funnily enough, like Xande, he too was cloned, but by Varis, probably concerned about what would happen with the war of succession in Solus's absence, which makes the reasons a bit similar to Amon's for bringing back Xande to deal with the empire's stagnation.
This isn't necessarily G'raha we're dealing with, but a time traveler version of him does seem plausible - could also be something else, like a clone. Either way, a connection to Zodiark remains a possibility. A vessel of Zodiark would be an interesting prospect - also not impossible, since Elidibus plays a different role to Minfilia in his capacity as Emissary, and isn't her Ascian equivalent, leaving the role open to someone else, but I was expecting that to be Unukalhai, if anyone, which also acts as a reference to him being kept child-like in the Ivalice universe. The way he throws the question at you, as to how might you accomplish your goal on the First, when you're a Champion of Light, almost seems like an open taunt. It just seems like a fitting destiny for him. A link between this Enigma and Solus would certainly possible, given Ascian involvement in the Allagan Empire, however I'm reluctant to draw too much from a similarity in the outfits, since it could just be the usual Garlean love of all things Allagan - and Solus is no exception there, Ascian though he may be.
On another note, the Tower presumably has to be powered by something to allow it to function as a sustained portal to other worlds - if that's what it will be - and I'm guessing that will be whatever power source Nero had hoped to find beneath the Tower, which his discarded device picked up on. Probably won't require as much power as sustaining the Voidgate took, but still a considerable amount.
Emet-Selch's star sign being linked to Gemini is something I hope that they will embellish and draw on a bit more, since they do appear to inspire the Ascian Overlords' personalities a bit from their associated FFXII espers, but on a selective basis and only in certain regards. I've no doubt they'll make use of the Death Seraph aspect.
All that said, the destination of the expansion has me pretty psyched... the cutscene toward the end of 4.56, rising up with the Tower, evoked some old FF nostalgia, especially for IX. I'm hoping that the actual method of travel won't require the Echo and will allow travel between the worlds, assuming the end-game isn't them merging with the Source (rejoining them but without the destruction the commonly understood method results in), in some capacity. The interesting thing is that this time, the Tower will actually serve as a beacon of hope on the First, and fulfill that purpose.