I do not know the level system, can you provide some examples of pieces around that level you have already played, and we might be able to help you out!
I'm not sure which Nocturnes your teacher was thinking of. I think that almost all of them are more difficult than that Brahms Intermezzo. However, the Nocturne Opus 15 #3 in g minor is relatively easy. There's one bit right before the transition to the B section that has a lot of chordal jumps and might take you some time to get comfortable with. It's a beautiful piece. The Opus 9 #2 Nocturne in Eb that everybody seems to pick as their first Chopin Nocturne is beautiful, but it's a bit harder than the one I suggested and you might already be tired of hearing it so much.Among the Schumann Scenes from Childhood, Of Foreign Lands and Peoples is well known and certainly within your level. I also like #12 "Child Falling Asleep," which is also not too difficult.For Bach the Prelude in C# minor in the first book of the Well Tempered Clavier is dark and beautiful. Shaping the lines well is a challenge, but finding the notes should not be too hard.From Janacek's "On an Overgrown Path," the section "The Virgin of Frydek," is a lovely mix of light and dark and not too hard. There's some 2 against 3 in there that will help you get ready for the Brahms you want.Slow movements from some of the Haydn Sonatas might also be dark and romantic enough for your taste, and many of them would not be too hard. In any case you can get inexpensive but decent Dover editions of all the Haydn Sonatas; lots of the earlier ones would be doable.For Bach inventions, the C minor 2 part invention is great, minor mood, long flowing lines. Another Bach choice would be a Sarabande from one of the French Suites - the C minor one might be up your alley.