My inclination would be to enclose that space using cabinet doors that look like paneling -- don't look like doors -- and add hidden storage -- usually much needed for items you seldom use. Alternately, you could just leave it blank because dusting anything put there would be a real chore.

If you do want to decorate that space, do you have a hobby/interest that you could hang/display there as art?

A actual bicycle from your childhood or bicycle metal wall art.

http://majestichondasouth.com/bike-wall-decor/bike-wall-decor-for-ingenious-ideas-metal-bicycle-getting-mine-up-soon-living-spaces-pinterest/

A musical instrument you've kept but replaced -- and/or metal wall art with music theme:

https://www.google.com/search?tbm=isch&sa=1&ei=MAqqWuuPMMyzzwL6vLPoBg&q=musical+metal+wall+art&oq=musical+metal+wall+art&gs_l=psy-ab.3..0.43606.47602.0.48360.16.16.0.0.0.0.117.900.14j2.16.0....0...1c.1.64.psy-ab..0.2.214....0.ZIt8kSfiTLg#imgrc=ugGJTgZ8hkCycM:&spf=1521093223693

Items you've collected that you seldom if ever actually use that could be hung on that high wall: Baskets. Cast iron pans. Plate collection.

Ditto.

Mirror window to give illusion of architectual elements

Two or three vases, but the tallest one should be 2/3 the height of the wall in the corner. Otherwise, they'll just be lost up there. You may need to shop for floor vases that are much larger than average.

Leave it empty.