What you already have on the mantle above the TV look fabulous , is it possible to hang them horizontal? Leave a bit of space in between when you hang them up . For the mantel it self keep it simple you already have the stone speaking for themselves more would make it clutter :)

I think that this one would look nice

Bone Inlay Wood Opium Square Coffee Table · More Info

I'm not a fan of mirrors over mantels, or anywhere really. I figure why look at a reflection of something you can already see? I'd suggest you find a great piece of art you love--pretty much any color since your room is pretty neutral--then coordinate your new sofa pillows to it.

Your current candlesticks and vase can stay. They're good looking and well-sized and totally fit the space.

Blue would add a nice counterpoint to all the earth tones in the room. I think a glass-topped table like this one from Crate and Barrel would look great, allow your rug to show, and offset the heavy look of your ginormous sofa. All pillows below were chosen to go with the art and are from CB2.

BTW I repainted your room.

https://www.greatbigcanvas.com/view/golden-day-turquoise,1166483/?s=haqLAkqRpaqrQG5hoaPDz3NVQogLiOPsEocp8RMrRXZVet10udkbSdhm3mGyG

all the mantle needs is to lean a single large piece of framed art. make sure that the colors coordinate but more importantly that is brings some personality and style into the space...no generic looking art, ever! check out art.com. and look at abstract art prints like: 'abstract sea 1' or 'deep waters 1' or 'opal' the art must be able to stand out from the beautiful stone work.