As was revealed during the live letter we'll be getting one new dungeon in 5.1 namely The Grand Cosmos.
I was discussing with friends during the PLL and had some immediate thoughts but then reanalyzed it to get a better idea. I wasn't able to see the screenshot until much later so the only thing I knew was the name.
So my initial thoughts based on the picture really brought me to Eulmore. The architecture for the doors looks very similar to Eulmore's large imposing doors, and the fancy hallway itself really beckons the inner workings of the area.
Mind you these were my INITIAL thoughts, but after taking the time to reconsider where we are in the story I had my thoughts split into different possibilities.
1). The dungeon is set in Eulmore and involves us going to the deeper levels of the city. This would hark back to Thancred in the trailer for Shadowbringers where he's going through the stairway under the city. I'd assume where the story takes us (mixed in with the screenshot of Ms. Chai) that we'll be involving Eulmore in the story for this. The one problem I can see relating to this theory is the image they posted showing off light glowing through the windows, relating to the area being outside. To that end I'd like to assume that there are more sin eaters down under the city and that they may have some specific ray of everlasting light glowing through the area relating to the Sin Eaters.
2). The dungeon is set in the Crystarium and has us searching for a resource to assist in sending the Scions back. As we know from the end of 5.0 the story is currently having the Exarch figure out a way to send the Scions back to the Source. I'd say comparing 4.1 having us search for treasure, the Exarch will have us go to this location to find something. Considering how the Crystarium has a lot of interstellar/space naming conventions and the theming of the Exarch's main hub "The Ocular" I'd assume the Grand Cosmos has some relation to it all.
Any thoughts?
EDIT:
The site has been updated to reflect 5.1 and we have new information on the dungeon.
"Long ago, on the far shore of the Source, a great palace was built by the elves─the Grand Cosmos, an edifice of such magnitude it was said to inspire envy in both the heavens for which it was named and the flowers that pervade its boundless gardens. As with so many wonders of the world that once was, its radiance quickly faded in the wake of the Flood, its halls falling silent, the throne sitting empty. That is, until the palace was given new purpose when a mage claimed it as their sanctuary, their only company being solace, solitude, and the legion of familiars barring entrance to any foolish enough to seek an audience"
The dungeon is taking place near/in Lakeland and involves the Elves which are a big part of the history of the fates in the area.