Schubert wrote many duets, most famous of which is probably the Fantasie in F minor (D.940) and the Duo in A Minor (D.947), neither of which is easy, but it sounds like you're up to it. Of course Schubert wrote a lot more duets; I believe Dover has published them in one volume.
Richard Strauss: Concert Waltz arranged for piano duet by Otto Singer. I see there is one for sale at Amazon right now. It's a very charming compilation of Der Rosenkavelier waltzes.
Mozart wrote several sonatas for duet: K.381, K.358, K.497, and K.521...all fun and charming.
Brahms: His 21 Hungarian Dances (WoO1) and Op. 39 waltzes both have 4-hand versions (by the composer, I believe). Then there are the Liebeslieder Waltzer, Op. 52 & 65. They were written for piano duet and 4 voices (SATB), but Brahms wrote them in such a way that the vocal part can be left out and the waltzes stand very nicely on their own. And they are not too hard.
To Debussy I'd add the duet version of the 6 Épigraphes Antiques...wonderful very late Debussy.
Dvorak's Slavonic Dances (Opp. 46 & 72) exist in duet form and are very nice.
Rachmaninoff, 6 Morceaux, Op. 11. Not all of them are super difficult and I like them a lot.
Satie: 3 Morceaux en Forme de Poire has a duet version. If you like Satie. They're fun.
There's so much to choose from. These pieces are the ones that come to mind for me.