Why does it need to be so large usually returns are just above the baseboard and wide but not so tall.

Maybe a frame with decorative lattice?

I agree with hubby on all the pictures and the lack of coverage of the vent...there were some good suggestions on lattice front tables in your other post..maybe have something built

Would it be possible to just leave the wall with nothing on it or in front of it?

Maybe a column of lattice floor to ceiling ,,like an architectural feature..wide enough to cover the vent...paint the wall behind black so the detail disappears

Maybe 3-4” from the wall?

I don't understand the resistance to putting a piece of furniture in front of that, not tight up against it, but cover it visually. The physics of air flow seem to me to allow air to be pulled from any space not actually physically up against the vent. That's why we caulk and weatherstrip, because air will seep into the tiniest spaces. I would put a larger piece of furniture there. if you are paranoid about air flow, then make it a console table and lean a painting over the top part that would come above the table. The bottom of the vent would be fully exposed.

oh, I like that cabinet with the doors. thanks for the ideas

I would paint the vent the same as the wall color first.

I like the first image but instead of that lovely object in front of the air return (doing a rather poor job of "hiding" it, I would suggest leaning a painting (canvas or framed) in front of it. Maybe move that object over where you now have that two-headed lamp (?) and move that elsewhere.

I'm still working on this project... will update when finished. thanks for the ideas!