4e had an excellent cosmology. It was resonant with reallife archetypes, and well thought out for use in a game.
Personally, I really liked the dawn war and the primordials. Though there is reference to the Dawn War, there is no reference to the Primorodials (except as a language for elementals) themselves that I have found. I could possibly make new ones, but using the ones that existed in 4e would not be allowed (I think). That, to me, is unfortunate.
If Forgotten Realms via the Players Handbook, mentions the ‘Primordial’ language. Then Primordials are canon. Use the term in the sense of the most ancient Elementals, the language that they spoke, and those alive today who preserve their traditions, including the language.
Similarly, ‘Anglo’ can be used as a term to mean anyone who speaks English, whether in Mumbai or Miami. ‘Franco’ can mean anyone who speaks French whether in Montreal or Paris. And so on.
Compare the Celestial as a name. Altho Celestial refers to the language, Celestials also means those who are native to the Good Outer Planes. Infernals the Lawful Evil ones, especially Devils. And so on.
Similarly, Primordials are canon, and you can refer to them as such. (Maybe even say, Primordial is the language of the Neutral alignment, especially as Elementals exemplify it.)
But there are specific defined parts of the PoL setting that it would be nice to be able to reference. The Nentir Vale was a specific area that was fleshed out fairly well and I think some would like to continue to expand upon it. Or set adventures or explore characters and locations during the height of Nerath, or Arkoshia, or Bael Turath. That is what some want and your suggestions don't allow access to the Nentir Vale or its history in the Forgotten Realms as far as I can tell.
The Forgotten Realms is a setting that has archeological perspective, with a timeline that covers tens of thousands of years. Moreover, Dragonborn and Tiefling are canonical races in FR. They have histories. Likely that had notable histories on one of the other continents. There can be entire empires on these other continents existing simultaneously with the kingdoms on Faerun, but with minimal contact with each other.
Track down the FR information about Dragonborn and Tiefling that is canon. It seems not much info about them. Try to incorporate this info as ‘facts’ in your setting. Then use 4e for inspiration for the histories that happen before, after, and around.
For example, in the 5e PH, it specifically says that Tiefling ‘lack a homeland’. Tiefling can show up anywhere as a Half Devil or a Half Demon. But there is plenty of room for a civilization that made some kind of Warlock pact for power during a war, which transformed an entire kingdom, and ultimately brought about its selfdestruction. This is similar to the history of the Drow. Except where the Drow went into exile underground and then transformed. The Tiefling transformed and then were scattered.
According to FR canon, the Dragonborn seem to be a ‘designer race’ resulting from magical alteration, ‘hatched from dragon eggs as a unique race’. Seems to me, Dragons did this for their own Draconic purposes. It is likely Dragonborn had their own civilization on one of the other continents. Create their FR histories, while drawing inspiration from 4e canon.
Personally, I would make up new names for these other civilizations. But if it is important that other players recognize the connection, use a lampshade name. Something like Ariksha and Baltur. Then, when the 4e setting becomes available, you can update your product to say something like, ‘in some dialects, Ariksha is known by its more ancient cognate Arkoshia.’
