That's weird. If it had an outlet it would make a good landing pad for keys, purse and phone. Maybe just a covered basket.

It's a perfect place for some primitive wood sculpture

That's kind of scary.

Glass shelving would be nice. Or, and not sure if this is your style, a tall vase with greenery or flowers or just a large pottery piece.

My style is kind of modern farmhouse (yes, I watch too much Fixer Upper), so I'm not sure the "unique" statue would work for me, but that's a cool idea! lol

I bet that could be removed and put back into side wall. I would investigate that first. TERRIBLE spot. But given that would involve construction all you can do is put a interesting tall object of art there. Something that would attract the eye away from vent.

Fill with a set of white shelves spaced well apart, with a floating corner, LED under lighting, and put all of your prettiest serving pieces or tchotchkes there.

There are many decorative options for your corner. It would serve a great place for a large object d'art....something that fits in with your style of decor, a large floral or non-floral arrangement, or additional shelving if you want to display more art, pottery, family portraits. I'm adding images of arrangements for inspiration. You'll have to decide what you'd like to see the most in that corner.

Without construction I think the best option would be a pot with greenery that hangs down I little with a tall narrow mirror on the side wall above it. A decorative iron grate will draw more attention to it, why not just paint it to match the wall color and make it disappear. Taking the wall all the way up with built in shelves facing the foyer is the best option, then the decorative iron grate would make more sense. If you didn't want to have to alter the crown molding you could leave the wall a foot or so down from the ceiling and it would still work.

I would paint that return vent same color as wall so it disappears, the iron will make it stand out more and then add a vase or 2

I would just frame it out to the ceiling....it would just look like part of the wall to the left.