i hear you about the people excited about Mercydia but i think we know too much about it as is, which is why it confused me when Hades brought it up because like what more is there for us to possibly know that hasn't already been revealed or is common knowledge at this point? how would they translate it all into a compelling narrative thats supposed to span across an entire expansion?
like i tell you what we all know about Mercydia if that will help my argument here but to give you the cliff notes version it boils down too: "people really hated the Allagan Empire so they joined hands with the Dragons to fight them"
but here is the lengthier part of what we all known about Mercydia:
We have to first go all the way back to Midgardsomr an how he fled his star cause it was under threat of invasion by the omnicron he took his eggs after having bested Omega in combat and fled too Eitherys which at the time was described as hovering in some kind of space-time bubble which Midgardsomr dived into traversing what we know as the inter dimensional rift an settling himself into the refection with the most aether concentration "The Source"
and its here he was confronted by Hydaelyn who granted him stay if he did one small favor for her an that was to guard the gateway into the aetherial plane to which he agreed too an was even given the ability to consume aether to prolong his life after death.
moving forward his children hatched an were taught about the lands of Eitherys and its people and then when they matured they all flew to the corners of the world.
Niddhogg, Ratatoskr, and Hrysvalgr went north and settled in the Dravanian hinterlands an abalathia's spine. Vtra and his sister Azdaja went east an settled in thavnir tropics, then Bahamut and Tiamat eloped an settled south in the continent of Meracydia
Bahamut and Tiamat were viewed as god like they were the figure heads of Meracydia and were the island's protectors/gaurdians we know there were beastkin there the tree like people are one example and the lizard-chimera are the other both sentient an intelligent races of people then later came hyuran an others who were fleeing from the oppression of Allag they sought refuge in Meracydia an Bahamut granted it too them they co-existed in peace but as we knew Omega had pursued Midgardsomr an crashed landed near cartineu flats where the Allagan's found it an brought it back to normal function an even found a way to control it remotely they began to push into the southern continent an this prompted the people to act they begged for Bahamut to be their shield and champion and he did he successfully repelled the Allagan's but they wouldn't give up so easily an thats when Amon orders them to release Omega an it defeated Bahamut crushing all of Meracydia's hopes this as we know of course was all by Ascian design they wanted to create this conflict in hopes of triggering a rejoining event so they approached the grief stricken Tiamat an the people of Meracydia an taught them the art of Summoning to call forth their own gods an this how the elder eikons were born in the forms of "The Warring Triad" and Bahamut Prime.
but this only threw the nation into chaos an the lyrics of Sophia's theme song tell us the tale of blood being spilled in order to maintain order an force away the Allagan Empire but this too failed the Allagan's were just far too advanced for their magic Omega defeated them all and captured them an they were then used for ghastly and inhumane experiments and a local beast known as "ixalion" was used to summon Garuda an later fused with more hyuran like genetics to become what we know as the ixal.
Tiamat cried out for help from her siblings however after witnessing the fall of Bahamut a second time an her sister Azdaja answered her cry an flew to her aid along with Vitra. they two attacked Azy Lla creating destruction and chaos but the Allagan's employed voidsent which we know the pair of great wyrms had already been fighting before on their own home island in thavnir. Azdaja put of fierce resistance but eventually fell into a trap an ends up going to the Void a distraught Vitra then is unable to continue fighting an he flees back to thavnir an Tiamat is recaptured an binded in Azy Lla but their attack did have one effect in their favor an thats that it created a window for the goddess eikon Sophia who used that opportunity to wake up Bahamut an this is what causes a calamity and sinks the Crystal Tower.
So with all this in mind i struggle to see how they could create a solid enough or compelling enough story line when so much is already known an the premise is just going to devolve into either Allagan shenanigans or Ascian's Fault for the situation all over again.. this why i thing Meracydia would probably be better off as a stand alone main quest rather than a whole Expansion worth of content cause its gunna feel like a Heavensward all over again an i dont think people want that.