Just posting this as a fun "what if" kind of thing, so don't take any of this way too seriously, just think this would be amazing if they ever made it happen. I was a huge fan of the NieR Automata raids, I still go back and queue for them sometimes even though I already have all the glams and have no actual need to run them, they were so good (in my opinion) that they made me go out and buy Automata and play it for the first time, which then lead me to play Replicant, 11/10 games. Obviously SE would never make another full alliance raid series just for Replicant, as much as I would love it, but a Trial in the same style as they did for Monster Hunter? Maybe that isn't too far fetched.
(NieR Replicant spoilers ahead)
Since The Tower at Paradigm's Breach already had Jack and Hansel and Gretel as bosses, the boss from Replicant that I think would be the best fit would be Hook (the one you fight for the first time when meeting Kaine.) It's probably the most important boss in the game outside of the Shadowlord and could easily be translated into a FFXIV boss. A trial like this would also be an excuse to add more glams, orchestron rolls, or minions to the game. (The Great Hunt had 3 orchestron rolls, 2 sets of armor, 2 minions, 2 furniture items, and a mount associated with it.) Have Hook drop a totem/token like Rathalos does and have it be exchangeable for glams (Brother Nier's adult outfit as well as Father Nier's outfit come to mind as good choices.) Have an orchestron roll drop from the trial (very biased but I would say having "Kaine - Escape" as the roll would be amazing, great boss theme, faster paced and has a more heroic/hopeful vibe to it compared to the version we got from the third Automata raid.) Maybe have the tokens buy other orchestron rolls, though no songs immediately come to mind, maybe an alternative version of Song of the Ancients? Not to mention I think a lot of fans would have to have a Grimoire Weiss minion (if we have room for two, maybe have a Wind-up Emil, or maybe just Emil's head rolling around behind you similar to what you see after ending B of Replicant.) Not saying it has to follow The Great Hunt 1 to 1, just trying to point out how a trial like this with so much loot potential wouldn't be unprecedented.
Sorry if this post comes off as a bit rambly, just wanted to get my thoughts out there, and maybe if the stars align, maybe this would get seen by a dev (or, more likely, it'll get 2 comments and immediately buried.)
Edit: Or people will take this post as a personal insult. Guess that works too.