I'd love to see the angst of guilt from Sherlock after he's done something terrible to John. I don't care if he's permanently damaged John or not, or the circumstances at which the damage occurs (but I prefer some psychological impacts, cos those are just more fun ^^). As long as it makes Sherlock feel incredibly guilty. Kudos for lots of Sherlock berating himself and trying to make John better (apologising, grovelling, lots of grovelling). Happy ending or not is up to the writer.