I do have a speculation about the beginning of 6.0: we'll be heading to Old Sharlayan first, and then due to their intransigence or perhaps some emergency, we'll go to Radz-at-Han. I think the "emergency" possibility is less likely, because in the Endwalker trailer, Urianger and Thancred are apprehensive about a
coming disaster, but one which has not yet happened.
As for why we will go to Old Sharlayan first, there's a picture from the recent Fanfest (and probably before that, but the screenshot I have is from Fanfest):
(Spoilered for size.)
That's a Sharlayan-style aetheryte, in a location labelled as Thavnair.
The first thought I had was that this may be a Sharlayan enclave in Thavnair, perhaps in Radz-at-Han proper, or perhaps some short distance, and thus we have to go through a zone story to get to Radz-at-Han, the way we go through Kholusia on the way to Eulmore.
So if this is indeed a Sharlayan enclave, we might be going to Old Sharlayan first, get stonewalled by the Forum, and then some sympathizers contact us and recommend we go to Radz-at-Han for some reason. And this Sharlayan enclave in Thavnair is our initial support base there, allowing us access into Radz-at-Han with some diplomatic weight, instead of just being travellers and tourists.
Admittedly, I could be wrong. The biggest point against this speculation is that the buildings shown in that screenshot are very much Thavnairan in style, rather than Sharlayan. And we know Sharlayans like to basically copy-and-paste their architectural style anywhere they make a presence, like in the Dravanian Hinterlands and Idyllshire.