Predictions for 5.3?

[Archwizard]

1. Its msq focus was far greater than Meracydia in HW, Meracydia had Tiamat's dialogue only even the warring triad was not mentioned as Meracydian until side story with Unakalhal

I wouldn't say any of Sharlayan was "focused" on in Heavensward. Tangentially referenced in many cases, but no more of a "focus" than Meracydia.

If you're going to use things as vague as spells and mottos as evidence of a "focus", then you could just as easily argue that the entire zone of Azys Lla is a huge reference to whatever the Allagans were doing in Meracydia, that Mhach is a good example of what the Voidsent the Allagans unleashed would have done to Meracydia, and that Midgardsormr and the First Brood make up a central part of Heavensward's story, which are a lot more substantial than many of the references you're making.
We also learn from the Warring Triad story that Sophia was summoned by a convocation of Meracydia's civilized races (including whatever race the Demiurges hailed from), and from Tiamat herself about how the Ascians manipulated her brood into summoning Bahamut, bringing Dalamud back around into the story. It's unlikely the Allagans shoved the entire dragon population of Meracydia onto one vessel, and quite probable they still have a handful waiting for Tiamat's return (which could make another solid substory and potential beast tribe).

I would also like to point out that the Warring Triad, Alexander and Mhach were not directions many people would have expected out of Heavensward just based on knowing we were going to Coerthas and getting involved in the Dragonsong War, much as Ivalice and Val were for Stormblood, or NieR was for Shadowbringers. To imply there wouldn't be enough side story content is really grasping for straws.

2. Meracydia's size is as big as the 3 great continents, this fact alone screams that it would take multiple expansions to deal with what Meracydia can offer and I would want that, Meracydia has a lot going for it and I would want it to have the love and care Ishgard and the 1st has had.

First I'm hearing of it being that big. Last I heard all we knew about its geography was "it's south of Eorzea".
We know it put up a massive resistance to the Allagans but that doesn't necessarily have anything to do with landmass. It's not on any recent maps, but on renders of the planet it appears to just be one big island smaller than Aldenard. About the size of Eorzea itself, really.

Bear in mind that:
1. Individual zones from expansions are bigger than zones from ARR, largely since they were built with flight in mind. Norvrandt is canonically the size of Eorzea, and we explored all of its relevant landmass in 5 zones. (Though it is worth noting that there appears to be a huge amount of space left to cover just on the island of Kholusia, given the NieR raids, and that our characters travel quite a while between zones within expansions, none of which is rendered.)
2. Othard is an entire continent unto itself, and we've only explored 3 zones of it; some of the largest expanses of it, like Rabanastre or The Burn, were relegated purely to dungeons. Even Coerthas only gained 2 zones in Heavensward, with the rest of its expansion covering Abalathia's Spine and Dravania.
3. We know next to nothing about Meracydia itself or how badly the Allagans scarred it, giving the devs complete freedom for what to do with it. For all we know 90% of Meracydia is Burned and we would only hypothetically explore a fraction of it. Even so, an expansion's worth of zones and dungeons would still be plenty to cover its relevant mass.

At any rate, you could use the same arguments to delay Meracydia indefinitely, "It's too big to explore all in one expansion," just as you theoretically could have with The First before Shadowbringers.