Another photo!

I love black, so you are preaching to the converted! I also think it makes your staircase stand out and the wood is warmed by the black behind it. The only thing I puzzle over that join in your second picture which seems so strange. Could you not paint over that in one unified line (or wallpaper it?).
It seems you are asking to lighten it up, but I think introducing white over the stairs will detract from the black and change its look entirely. Could you not hang two sizes of your Ikea dandelion pendants in front of the stairs at staggered heights just to introduce more contrast (but then you will loose your filigree designs that you have placed everywhere?

I'm happy to say the light fittings, including the idea light up on the mezzanine level are earmarked for replacing. And the ceiling pine panelling up there, which seems so at odds with the rammed earth walls elsewhere!
On an aside, the wall art is temporary, simply remnants of a bed head I deconstructed and a door screen : )
As for the joined bit in the second photo, here's a better view- hard to know quite what to do with it. I love the suggestion of wallpaper in that I could make it into a feature rather than a question mark lol
Loving your input!

white is too traditional for your home. how about painting every 4th spindle black.as budget allows,you can replace them with salvge iron pieces. i'd like to see more pics of the house. it sounds like you want to keep the fun, cool character so you'll have to guard against main stream fixes.

and completely unsolicited but i have to ask:
are you planning to change the space under the satirs? the dresser doesn't seem to make seems out of place there and the shape is wrong.it's a perfect spot for bookcases or to artfully display a collection.

DIAspoton, great suggestion re every fourth spindle, I'll have fun working more on that idea!
It's interesting reading responses and also taking a fresh look at the stairs via the pics I posted. They aren't really as bad as I thought! Perhaps my bias was residual from when I first moved in and the walls behind we're a hideous stencilled green and orange.
I'm still trying to work out what to replace the current wall art pieces with, it's such a vast space.
And I'd be interested to hear more alternatives to the dresser under the stairs. Strangely it looks more in scale for the space in real life than in the photo but I do agree with you that it is not quite right and would love more suggestions.

under stairs : a basket in the narrow end for hats, ball caps,dog leashes. add a bench with room underneath for shoes and boots and a boot jack. install hooks above for jackets.
OR : dog bed and crate with framed poster above
OR : a bar set up
OR : a phone chat nook
OR : a reading nook with book shelves , a comfortable chair and light
OR : a statue. check resale markets and garden centers for large, outdoor concrete statuary. you might find something fabulous to showcase. put some moss and rocks around the base.

as for the large wall : you need just 1 large round object in the middle. take care not let it look like a bullseye!
the shape would break up all of the angles for visual relief and it mimics the window for continuity.
this could be a bright painting
OR : a metal starburst
OR : a industrial or automotive salvage
OR : painted or wood folk art
OR : a round 3d collage of items with meaning for you
OR ; skip the round and hang a very large, colorful textile. could be a rug, a contemporary quilt , a silkscreen , a weaving. shop local galleries, etsy