¡Y lo que te rondaré morena! is mainly used -I think- when there's a delay in something. For example, you need a plumber, phone one. One week later, your neighbours ask you about your plumbing. You tell them that you are still waiting for the plumber, you can "wait and see". Another situation is when you are up to your ears in work. Someone asks you,"Haven't you finished yet?" And you answer, "No, y lo que te rondaré morena." I think that the origin of this expression comes from persistent boyfriends who chase after their reluctant girlfriends "rondar". But then, I may be completely wrong.