I don't think your ceiling needs crown molding. That would be overkill on an already beautiful ceiling design.

Hi Kristyn, I can see your desire to add crown molding - it would reflect the substantial base molding and finish the room a bit more while accentuating the shape of the tray ceiling. The issue I see is with the return air ducts. If they could be relocated to the flat plane of the lower ceiling height, you could have a nice area to add a cove molding. Typically, you would then want to repeat the molding on the walls as they touch the bottom of the soffit as in this example.

With the fireplace, the niche is a little too deep and dark, not to mention in competition with the television. I would suggest adding a directional diffuser to that recessed light to give more of a wall-wash effect to help illuminate your decorative items. The simpler the better - say 3 striking hurricanes:

Or, if space allowed, fill in the wall and reposition the flat screen over the fireplace. And, thirdly, you could consider filling in the wall and creating a recessed wood niche to the right (at 90 deg to current tv wall).

This following might work as well although the wood pieces would need to be fairly short as you approach the sides of your arched opening.

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Good luck!