Hi gwahir - you really should
add your car to the garage so we know exactly what year/style of Geo Metro you're talking about. The doors on a 95+ hatchback are very different from one off a pre-1995 4-door car.
Considering their safety value, I'd say there are other places I'd look first before replacing my doors with something lighter, though.
If you were really hardcore and didn't care about safety or having a wind-up window, I suppose you could make a mold of the existing door and fabricate a fiberglass skin on a simple frame with a lexan window.