As I wrote to few ppl before in private - all I have this time is just a pack of screens, nothing more (and based on the screens themselves there is no point in guessing like with DF where I had much more information).
It is too risky to upload them in higher quality, as they may be sent intentionally (they may be, for example, generated by AI) to target people who post information during the NDA. Also, as I also mentioned - don't take it too seriously but: most of them have locations with purple skies and models that look very much like dreadlords.
That's why this time I'm refraining from posting anything, even in the same form as in DF - although it was very funny when you discovered the next clues one by one
I simply don't have much information (only a handful of screenshots), and I don't want to cause confusion.
Personally, without looking at any screenshots, I bet that the expansion may be focused on voids, because many things in the game would indicate this - but this is just my personal opinion. Before shadowlands, there were 4 things planned: shadowlands, dragons (dragonflight), light vs void (???) but also undergrounds.
In the case of the underground, it may also be different - because, as you yourself have noticed, there was mention of undermined somewhere, so Towellie's recent posts may point towards this - but whether it will be added to the game as an add-on or as a patch - it's hard to say.
I participate in speculations by reading, day after day