Steven Moffat, that old troll, said about Sherlock in this interview "maybe he marries Mrs Hudson". Yeah, as if that would ever happen - but, oooh, it would make a good fic!
Under what circumstances would Sherlock marry Mrs. Hudson? Of course it would be some sort of marriage of convenience. If I could write, I'd write something fun, not something unimaginative (like she's his beard) or serious (like at the end of Boston Legal).