How long do you want the piece to be? For something longer I would recommend any of the four late opuses 116-119, they are all great masterpieces. You could always choose one or two pieces from one of these opuses too if you don't want to play an entire opus. Then there is also the op 10 ballades which are wonderful while not quite as complex as late Brahms. Whatever you pick you can't go wrong. Every piano piece by Brahms is a masterpiece!
this is so rad, i not unknown but just doesn't seem to have the spotlight his other works have in terms of what i found other students playing when they did Brahms...worth checkin out , i love this thing, it's on my short list along with the transcription of the Scherzo from Schumann piano quintet