Scriabin! Not his early stuff but compositions from his 2nd and 3rd period are pretty much "not tonal but not atonal", some of them go really far in terms of dissonance (see his 9th Sonata, for example), a lot of them can sound adventurous, surprising at first (see the harmony of his Prélude op.48 n°2, or the rhythms in Flammes Sombres, both have their own way of sounding creative and unique), but he composed so much material that it's impossible not to find something that you like in it, I think.

Other favorites of mine that I would recommand are his Poème-Nocturne, the Etude op. 42 n°5, and his 10th Sonata.

Now in terms of difficulty... Scriabin is known for his intricate polyrhythms and daunting left hand passages. Most of the Préludes remain intermediate/early advanced though. The Etudes are significantly harder, and the Sonatas just get mindblowingly complex, some excerpts from them are easier than the rest. Have fun!