Whatever you put there, it's going to be there -- less noticeable perhaps, but still noticeable.

I'd look into calling an HVAC company and having the return moved. If you want the wall to be a showplace, it will be worth the likely not cost-prohibitive cost.

Unfortunately your return is in a bad location. Heed the advice to call your Hvac company.

"i think such a tear up is unnecessary."

It's not a "tear up". All she has to do is a drywall patch over the hole that's left after the vent is moved.

I would frame it out, paint it, and find a couple other thing with horizontal and vertical lines like flat woven baskets and incorporate all of them into the gallery of framed pieces.

Shouldn't a cold air return be on or near the floor, where the cold air is?

Would leave it as is

Is this a basement ? I almost always see cold air return near the floor where the cold air is .So the advice to talk to the HVAC company makes sense if this is not a basement .I am not a fan of "gallery" walls but I agree it diappears becuase you eye goes to the art so if it can't be moved just ignore it

re porkchop comment. theres a duct system behind the vent outlet. it will have to be rerouted elsewhere which means minimal tear up or major tearup depending on where the homeowner wants new vent outlet.

If open to changing the two larger frames to simple wood frames to create a more seamless effect….

….then add a wood vent cover and it will disappear. If possible, center couch underneath.

I would just add a frame around it. Whether it be wood, black, gold, etc.

I'm guessing at the size of the grille, but I think standoffs can be 1-1/2" to maintain air movement.

This idea might help and it might not, but it's fairly easy to do. Unscrew the grill, rotate 180 degrees and replace. Louvers might reflect the lighter ceiling and make the grill blend better. You might also be able to see into the duct, which might not be good.