Here's a thing I always wanted to see but am too busy to do myself: an explorable map of the world with different overlays you could toggle and a separate temporal scrollbar. Something I could use to see e.g. how the borders changed over time by dragging the scrollbar (and that would be accurate - e.g. took into account that nation-states are a recent invention and borders had a different meaning in e.g. medieval times). Or follow the battles of World War II all over the world. Basically, interactively explorable history of humanity.

I too have been wanting something like this for a very long time... I even have a couple of very early prototypes lying around.