I enjoy alot of Prokofiev's music. I would suggest his Sonata #8 as a great work-it's very long, but I would call it truly profound.
Then, there's his toccata. Right now I am polishing this thing up-it is very tricky, but I don't think it's quite as bad as it's reputation would suggest once one gets the feel for the rythms and movements of the piece. It just kind of clicked, a hard sensation to describe.
Bartok wrote some interesting music-I've not actually played them yet, but I have been listening lately to his allegro barbaro, his 3 etudes, and his sonata. All are fine, difficult works.
I have an affinity for the sonatas of the obscure composer Violette. They are definitely not easy listening, and not for everyone. Right now, I'm working on the sixth sonata-it's the most difficult piece I've ever attempted.
Maybe you could try the Berg Sonata-it's very Romantic sounding. Then, there's Ives' Concord Sonata, Samuel Barber's Sonata, and even the Feinberg Sonatas. Ornstein wrote a large corpus of piano music-I can give you a link to a website with lots of pdfs if you want.