In this interview with Banks' friend and fellow SF writer Ken MacLeod there's a brief comment about Banks having the germ of an idea for another Culture novel that he never got to write:
AL: Did Banks have plans for other Culture novels beyond Hydrogen Sonata? What might it have covered?
KM: He had an idea for a novel about a character who had stored some of his memories in ammunition, so every time he used his weapon he lost part of himself. He hoped to have left enough of an outline and notes for me to write something from if he didn't have enough time left to write it himself, but sadly his illness didn't even leave time for even an outline. It was a generous idea, and typical of Iain, in that he inisted he would like me to write the novel in my own way and not in a pastiche of his, but even so I think I would have found it almost impossible.