Why not some Chopin Etudes? I mean, you got though Rachmaninoff's Prelude in c sharp minor, so you clearly have the chops for it! They're extremely fun to learn (albeit, very tedious at times) and perform, and learning some these etudes will provide you with the skill set to play much more difficult music. Some more accessible etudes include Op. 10 no. 3,5,9,10,12 or Op. 25 no. 1,2,7, and 9. These etudes fit rather comfortably in the hand (well, this is compared to something like the infamous Op. 25 no. 6, with ridiculously difficult thirds passages), and are perfect for an intermediate to advanced piano student.