I would resist the temptation to try to retrofit any kind of mantel, and in fact I would not hang art in the space between the TV/fireplace and the windows above (possible exception would be a long narrow thing, like ironwork or an oar). The wall is busy enough, an dyou can hang art on the wall behind the couch, etc. I think I'd also paint the cupboard under the TV to match the walls or trim.

Only other thing might be to fill in the entire fireplace niche with stone, rather than just the outline of the firebox. Wouldn't do more of the wall than that though.

Sorry -- you'll find everything works better if you work with the style of the house you have, rather than trying to shoehorn it into the style of a totally different house. This just isn't a beam mantel fireplace house.