Stemming off the discussion of the Sinclair building getting a hotel conversion.
You can see this graphic from the historic commission clearly labels this lot as slated for a hotel and sidewalk improvements.
In Two of the renderings for the Sinclair you can make out parts of the possible building
Here at 5th and Main the lower floors of it are on the left.
And here you can see nearly the full height, about 11 or 12 floors, and seems to include a nice rooftop deck.
Assuming it would be about the same height as STS or Burk Burnett, it'd be a very thin building, about 50 feet wide.
Since the building is so well detailed in the above rendering and the other buildings are stock models from Google Earth, I'm thinking that the same group behind the Marriott Sinclair conversion is also behind this building. TAD furthers this, as the lot is shown to be owned by "Sinclair Holdings LLC", obviously the same group behind the conversion.