You certainly shouldn't do anything to obstruct airflow to either vent but especially the return. I might suggest that you replace the return grille with something more attractive. You could even consider putting a frame around the grille to treat it as artwork and then group other art around it to create a small gallery.

Move the bar to the left space of the fireplace with a round mirror above. This will fill the space with useful purpose and allow easier movement around the end of the dining table. Moving the sofa back, you could reposition the chair angled at the end of the sofa.

Can you paint the vents to match the wall color??? I would not place any art on that portion of the wall. Place art on the wall beside it…a big poster, etc.to draw the eye away from the vents, etc. Distract the eye. Right now, the white paint on the vents say “look at me!”

Paint the cold air return wall color

Un crowd the nook

Paint the brick on the fire

Get a larger and more interesting rug

Take more pictures when you are done mopping and the table set up is correct

Thank you! So no votes for making one of those three sections an accent wall or doing built in bookshelves?

Here are some thoughts and suggestions for pulling the whole space together, not just that wall. Good design, that makes you feel good, takes into account the entire space, not just one feature.

--No accent wall

--Get 10" deep white bookshelves to flank the wonderful fireplace--a great feature of the room. Wayfair has 10" deep white bookshelves. This will lighten the room, which is so gloomy because of all that gray. More on that later. White bookshelves will echo the white bricks in your red brick fireplace.

Wren Cottage · More Info

--If budget is tight, just get 10" deep bookshelves that are 36" high so you can use the top shelve sfor TV, bar, buffet, etc. Or get taller ones and run a mantel across fireplace to bookshelves like this with a great art work over it.

Classic Modern - Fireplace · More Info

--To echo your golden floors, get matchstick shades for your windows. Those gray curtains are a downer.

--As others suggested, paint grills same as wall color.

--Okay, let's get to wall color. The gray is so gloomy looking and doesn't work at all with your golden tile kitchen floors, the golden oak floors, or the warm red and white fireplace. That space should be entirely a warm white to echo your "givens" and make the room looked pulled together.

Layout fixes:

--Move your dining table so it lines up with the wall edge.

--Move your sofa back so it lines up with wall edge where you have an art work

--Move your left armchair so it tilts toward the sofa to create a conversational connection. Move the right chair closer to the sofa. Fronts of both chairs should touch the rug so you definite the space.

--Get a great chunky golden oak mantel and an art work in blues and golds to go over it, again to echo your "givens."

--When budget allows, get a much larger rug so all upholstered pieces sit on it.

--A white nubby rug with navy, gold, red accents will echo upholstered furniture, reddish fireplace with white bricks, golden floor, golden kitchen tiles, i.e. the rug connects to most everything in the room so it all works.

--

The best change would be to paint the walls a warmish white, painting the grills to match the wall and replacing the gray curtains. The gray doesn't work with the floor.

Here's a tall red brick fireplace flanked by white bookshelves--reminds me a lot of your room with warm floors like yours. Look how light and bright the bookcase and warm white paint make the room.

Thank you!! All super helpful comments. Will post when I’m done