"...and now it looks too dull."

To me, it doesn't look dull at all. That is certainly a rich, dark color on the wall. I love something we don't see often, like your blue wall. Also, I like a collected, gallery look on walls, but I think in your case it isn't quite working because you have unified the collection (that I can see). Also, some of the items don't have frames (Are they stickers? Painted images directly on the wall?), and are dark in color, so they don't stand out from the wall color. A great way to do a gallery wall is to unify a "theme" such as:

All gold and black frames

All mirrors

All old-timey prints

I encourage you to frame everything on the wall, place the items closer together (better to leave wall space AROUND the collection than space them out evenly. I'd place them maybe a 1 1/2" apart.

Finally, I can't help but add this even though you didn't ask. When I first saw your post, I thought, I'd decorate those slick-looking stairs with a banister. That looks completely like an unsafe staircase to me, someone is going to slip and fall badly.

And one more thing: You need some lighting in that stairwell. The slippery material and lack of anything to grab as one loses their footing is exacerbated by the dim lighting.

Mrs S Thank you So much for your comments and ideas ! And you are right that the gallery shall stand out to work correctly ! I’ll update when I made the changes . Thanking you , you are also right about the banister . We didn’t have one installed back then when we were building, as the staircase is narrow and the workers told us it wasn’t enough space . However, now especially with the new materials , we will add one for sure .