ah.. interesting post! chopin and liszt (along with beethoven) are my favourite!

i'll see 15 minutes as your preferable choice. difficult works, lets zoom into sonatas then! chopin and liszt's sonatas are all headaches. (apart from chopin's first i'll say) but chopin's sonatas are too long for your case... sigh. if you want to look into them, sure!
piano sonata (liszt) is way too long, approx 30 minutes. but the other sonata by liszt, the dante sonata (Apres une Lecture de Dante, Fantasia Quasi sonata) is around 15 minutes long! (but people play around there, some 14 minutes in blazing speed, others 18 minutes, while i take around 16 - 17 minutes.) it is among the hardest pieces in standard repetoire.
now big challenge. do you want to try the tannhauser overture transcription by liszt? (liszt's transcriptions, are horrendously difficult.) its really beautiful piece... good to the human ears, good to torture your fingers too! thats as much as i can think of now, so all the best!
i'll say look into liszt's transcriptions.