A few late suggestions. I'm also giving a Russian programme (in a couple of weeks) and among the favorite pieces I unearthed while chosing the music for the recital are Medtner's Primavera (op. 39 no. 3), the Kalinnikov Elegie and loads of pieces by Bortkiewicz, such as the Elegie op. 46, or the studies op. 15 (especially no. 4, a technically easy but extremely beautiful piece, in 7/8 time!, and the passionate no 8, perhaps the best-known of the set). There's lots more wonderful Bortkiewicz out there. If you play Rachmaninov, I suggest considering the only two songs the composer himself arranged for piano: Lilacs and Daisies; they make an exquisite pair and show the full-blooded romantic side of the composer to perfection. Lilacs is also quite a showy piece, and makes a great foil to the terrifying violence of the Prokofiev (what a piece!).
Personally I'd consider your choice carefully if you decide to perform a Medter sonata. I'm playing the Fairytale Sonata in my recital at the end of May, and it's given me more trouble than anything else in the program apart from the Godowsky Java Suite selections (not Russian I know, but that's another story). Technically it's doable, but most of the Medtner I know (including some of the short pieces) is thre devil to interpret, and it can take a long time to form a really convincing interpretation. At least that's been my experience; maybe the Fairytale Sonata, with its sectional form, is just one of the trickier ones to pull off. Fantastic piece, though.