Hmm, can't really suggest enything besides changing the BG, play more with colours and add some typography
maybe.
For the BG I thought some japanese interior would do, maybe some jpn sliding door (called fusuma if I remember
correctly) with black&white japanese painting on it. Could open them a little from left side and let some moonlight
inside... but the whole wall will have to be longer then, cause theres not enough space for scenic now I guess XD
thought of flowers too, but they wont fit if you wanna do a mysterious wall..
As for the colours.. using a pattern for her skin is kind of nice and interesting, it fits xxxholic^_^ I would use the
rose, make it more bright red and make the BG with blue and violet. You don't have to use a lot of blue to make the
contrast of blue and red be noticable. My bad I'm into the moonlight already^_^ If you wanna stay more in
monochromatic it leaves to put the red on the rose and maybe some in her eyes (she had red eyes I recall) and let it
have all the focus.
Now the strongest point if the wall is Yuuko's hair. Looks pretty nice. You can play with typoghraphy for the BG
too, but I can't help with that much.
Overall, you need to choose and add something catchy, like contrast colours, monochrome with red highlight or whatever
you can think about to make the wall more lively.
Good luck with it!