The set of photos below show how the soffit in the basement was "hidden". Here is the "before" picture:
I have discussed this in past - the two things that really detracted from the house where the long, dark, narrow hallways and the HVAC soffits across the ceiling in the basement. I really didn't want to go the expense to fully redesign these (if possible), so I focussed on hiding / incorporating them.
The soffit shown above became the defining location for the theater room wall. We were lucky in that the theater room size is fine (600 sq ft) doing it this way.
The pictures above and below show the new wall and the theater room entry it forms. The openings on both sides are for bookcases. I actually made the soffit a bit wider so I could put a row of recessed lights along it. These light up the game / homework table nicely.
Here is the rendered computer model, showing the final planned result.