I would like to be able to specify a different extrusion multiplier for the first layer. "Top layer" (meaning the top layer of any area, not necessarily just the highest part, which I'd call "Last Layer") would be useful too. The only way I know to do a different extrusion multiplier for the 1st layer right now is using multiple processes.
An alternative would be to add a way to specify that a process be for "top layer", rather than "above a certain height." So for stairs, for example, it would recur on the surface of each step. This would give the ultimate in control, especially if you could say "the top n layers".