I would have a mantel for the entire length of the wall

Do you mean sheet rocking the opening to the left of the bar to the room next to it? Not sure where the new cabinet is going. To the right of the FP? Or over the wetbar? Where do the glass shelves go? Onto the sheet rocked wall to the left of the wet bar? I'm sorry I'm having a hard time deciding about the mantle & where it should go, so I'm asking all of these questions.

Meanwhile you have gotten suggestions from JudyG & printesa & I am inclined to agree with their ideas, too. Do not add too much to the mantle when it's up.

I was visuallizing your FP as a kiva type which do not always have a mantle.

To the left side. Where the wet bar is now. Rocking in just the depth of the bar area.......so I can insert glass shelves with a light shining down through...maybe three shelves