More or less, every major nation starting from Inazuma has received three attached expansions, each of them meant to bring the Statue of the Seven to level 10 through the Oculi scattered across those maps. This doesn’t apply to the first two nations, since starting from 1.2 Hoyoverse introduced standalone offering systems for Mond/Liyue instead.

In other cases, we’ve seen previously out-of-bounds areas become fully accessible (such as the northern part of The Chasm and northern Sumeru), because the main regions were already complete. Areas like Mont Esus and western Sumeru are still unknown to the players, and they ofc don't know whether they'll receive special attention or simply exist as visually satisfying regions unlocked to fill out the map.

That said, Hoyoverse has been relatively clear about Mondstadt and Liyue from the beginning, especially Liyue, with areas like The Chasm, Chenyu Vale, and Blackcliff Forge. Mondstadt, on the other hand, has always had Dragonspine and Dornmann Port, while the Dandelion Sea was described five years ago in an eleven-volume book series as an endless field of dandelions inhabited by foxes, nothing else. And if they can build an entire place based on unknown to us information, good for them.

Realistically, it was mostly longtime players who speculated and treated it as a "main expansion" even though there’s currently nothing that really supports that idea (Purple Sea case). Personally, if we ever do see it in the future (and that wouldn’t be strange since a place in Genshin can become "real") I’d expect it to function more like a Domain like the Oasis released in 3.4 Patch. The Sea of Flowers at the End exists as well, it is not the same but we'll go there after our journey.

It’s also true that the game itself hasn’t said much about certain areas, but the difference in available information between a place like Blackcliff Forge and the Dandelion Sea is huge. Anyone posting leaks about the Dandelion Sea shouldn’t take advantage of how little information exists about it, especially when it’s even less defined than the Purple Sea.