What a party room, especially the fire pit -- fun! I can see decking that out in wood -- either dark stain or black paint, with upholstered benches, throw pillows, and a faux fur blanket. I'd go for your grout color on the walls, and arrange clean lined, super comfortable seating around solid color rugs to juxtapose the stone. Most of the interior flagstone floors I'm finding among houzz photos are rustic, but I think you can pull off a real casual modern swank with this space. Hrundi Bakshi would love this place!
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I love the furnishings you have there now, they are neutral and compliment your beautiful floor. But not the blue rug so much.. it kind of clashes with the flooring. I would suggest a more subtle, washed out, pattern in a neutral color, or no pattern at all in a neutral color.

Maybe pick up some of the colors in the flooring and use it in pillows, throws, pics/decor etc.

Also, if it is an option, I would trim out all your door walls and windows, doors and door ways with the same stained wood, or similar, that is on your ceiling. It would really dress them up nicely and feel more unified with the ceiling and flooring. The white trim just doesn't feel like it belongs.

The small sunk in area in front of the fireplace.. umm.. well that certainly is different. I can't even picture what I would do with that space but I would love to see your final reno of that area!! Please post pics when you do remodel it!

Beautiful and unique home..congrats!!