Don't use hotspots. Use semi-transparent shapes instead. Why? Because hotspots don't have States, but shapes do.
- If you want the shapes to be invisible to the user, give the fill a 99% transparency.
- Give each of the lower shapes a Disabled state, and make that the Initial State.
- In addition to the trigger to do whatever clicking a shape needs to do, add another trigger that changes the state of the next clickable shape to Normal.
Depending on what's on screen, consider adding a Visited state to the shapes, with some change to indicate that the user has already clicked that area.