It's probably worth remembering that they've said they intend to remove Trials from the MSQ and have Primals as their own separate side story, so it is entirely possible the MSQ wont touch on Azys Lla at all and Sephirot will just continue on from the Extremes questline, rather than being central to the MSQ.
If I had to guess, Antitower would be this games version of Eureka, I seem to remember Yoshi saying it was planned and "not quite what we'd expect"... I'd like to think the MSQ would follow through and mimic Crystal Tower in that case, just in reverse... Not just going down instead of up, but what was at the top of Crystal Tower? A Void Gate. One which threatened to flood the world with Darkness. What would be at the "top" of a hypothetical AntiTower? A Light Gate? A direct link to Hydaelyn? I'd like to think the whole "Silvertear Falls is the source of all aether and Middy guards it" originated somewhere, but perhaps that wellspring has been blocked, and that we go down there to bust the doors open and power up Hydaelyn (followed by "Now I have plenty of aether to spare! Please feel free to summon Primals to help you in your quests!", hey I can dream right? It's Christmas after all!).
Granted, we still have the whole "truth" Elidibus alludes to, which hints that Hydaelyn is teh big bad, but I fully expect that to be something like "Hydaelyn perpetuates a cycle of death and conflict! She wants us to suffer!", given the general theme in songs like Answers, and the fact that apparently the future has been written already... I fully expect the Ascians to want to end that, burn the book and live forever (would now be a good time to remind everyone of the faux-immortality?), while our heroes will accept death as a necessary part of life. We've already had one Scion accept their fate and die, actually two kinda, then we had another ally thank us for showing her the way as she died. Accepting death seems to be a thing in this story, and I'm starting to get an idea what Hydaelyn told Minfilia that shocked her so much as we fled Ul'dah...
As for Nidhogg... I'm not entirely convinced he is himself right now... Still holding on to Lahabrea surviving in that eye and influencing him in some way... What can I say, I really want another "I am become you!" fusion dance from the Ascians, but Nidhogg + Ascian this time giving us Shinryu and fulfilling the franchise canon of having Omega pop up to kill the dragon... What? How would you end the Dragonsong War? End of an Era would have nothing on the Omega/Shinryu cutscene my headcanon has created...
But yeah, before we close the book on the Dragonsong War, I think it would be important that we know what exactly is going on in the grand scheme of things, and Midgardsormr would be our best bet for that. AntiTower as part of the main scenario? Well it just so happens some of the biggest answers are hidden beneath Mor Dhona, where we have a famous Tower... One which famously had an underground... I'm just putting two and two together here, plus I really want the message this game is sending to be one of "Let it go", not for a cheap Disney reference, but because it is delightfully ironic, given this games history... Heck, the franchises history...