I think a very limited number of overlays would be a good inclusion - but as Joe say, "a little goes a long way". If after hypothetical SK5&EP3, there were a total of 3-5 overlays (inc RW1), that'd be enough - with design rules limited to one per scenario, etc. What sorts of overlays might be most useful? A funny challenge there is the limited terrain palette and the need to avoid funny combinations of terrain otherwise avoided in SK.
A Hill or two seem the most basic addition, particularly helpful for maps y and z. Beyond that, simplified "special terrain" seems best to me, to spice up scenarios with quirky objectives/obstacles - a small multi-storey building (drawing on DaE) or rowhouses, a stream, etc.