Often when I’m planning a VFR flight at a certain altitude I find it hard to tell the Height of the landscape even when zooming in on the current World Map, often I’ve created Flight Plans on the World Map and had to alter my altitude multiple times during the flight. This of course is not an issue in itself, but a Heightmap Layer on the World Map could be useful for planning Flights, especially through Valleys/Mountainous areas.
Seconded, I was just thinking of this one.
Skyvector has MSAs and terrain contour lines for VFR flying, you can plot and follow your route on there to determine your Minimum Safe Altitude 