Like just hanging a cool door (maybe antique) from the wall like art?

Maybe. Or interior windows....faux or even real depending on what is behind the wall. Could be made charming with shutters, window boxes, even little rooflet if you really want.

I would consider just some wood paneling or millwork to break up that big wall.

Nope. Art gallery of big good quality art elements. Lose the tree murals.

No, what to do on landing. Don't like the trees up there.

Are you asking about the area where the trees are? What would you think about painting that wall an accent color? Do not paint the are below where the white door is just the large wall. A warm apricot color maybe. This would draw attention to the beautiful windows above and the bring out the wood tones. What a gorgeous space you have.

Got kids or grandkids? I see there is a ladder, so I'd put railing around it like bunk beds have and would add a bookshelf at the opposite end, a nice, soft upholstered cushion to fill the space, pillows and call it a hide-away reading nook. Here's a colorful example that could work up there.

If you are asking about the large expanse of upstairs wall above the vertical beams on the left side of the image, I agree with the suggestion to put very large, high quality art there.

What is behind that wall?

Architectural glass wall? Allow lite but add some texture and interest depending on design you choose. If you want to go all out. Put a water feature running down the glass wall. Then it would be a WOW focal point

lose the trees. put a large, interesting sculptural form up there. could be driftwood, an old industrial machinery piece, anything 3-d and artsy. then use that fabulous wall with large art as others suggested.
everything on the wall now looks dwarfed. start with one important painting and add a few lesser canvasses and textiles to take advantage of the space.

Thanks everyone. Yes I have grandkids, but don't want them up there. I do like the rod iron, remove ladder idea & paint wall ideas. The items there now are sellers. Planning adding an lrg entertainment area to fill wall. Also like lrg art idea. For entertainment center, wood or stucco?
Thanks! More requests for help to come.

Oh..& there is nothing on other side..outdoor wall.

This is what I was thinking of for the wall??

Wow, what a magnificent space! You need a Jackson Pollock art piece--or more--in there! You gotta think BIG.....

Well, I think I'd extend that loft from the point where the ladder is, to all the way into the corner of the fireplace wall. Then have a really great pair of spiral metal steps coming down that look like a piece of sculpture, very industrial feeling.

I would not put shelving on the walls. I would put those strips of metal on the studs so shelf brackets could have long wide planks added whereever you like, really open shelving and not book cases. If possible, somehow I'd look for a way to have an automatic stainless steel beaded hanging curtain wall like a stage curtain just for the sheer drama of it. Elegant silk hanging panels with aircraft themes would also be harking back to its look of a hanger.

What a place to live! Does it echo? Maybe if none of that appeals to you, you might cozy it up a bit with a hanging oriental rug, perhaps extending across the floor to bring the wall dimension down and tie it more securely to your flooring. And to YOU.

The door is good antique shutters a book case but u would need a ladder