I'd be surprised if this hasn't been baked, but wouldn't know where to start looking. This idea is based on a modular gear ratio system.

Imagine an epicyclic gear ratio (see link), taking either direct drive (ring and planet carrier locked) form or reduction (ring locked to housing, drive from planet
carrier) form. This would be implemented by having the unit naturally locked in direct drive mode by a sprung system, effectively pegging the planet carrier to the ring gear. The 'reduction' mode would be accessed by having a band around the ring gear that, as it clamps down, releases the planet lockers and then takes hold of the ring gear, fixing it to the gearbox casing.

Now imagine two of these running in series, but with subtle changes in the tooth counts to give different ratios when in reduction mode. This gives four distinct overall ratios... D-D=1
0.9->D = 0.9
D->0.8 = 0.8
0.9->0.8 = 0.72

Using more ratios would lead to 2^n possible overall combinations (ie. 3->8, 4->16, 5->32). Since the most basic implementation if this could be implemented using n digital outputs feeding relays, valves or other actuation equipment. If you were feeling particularly clever, you could have the switches for each stage on the dashboard and flip them manually (assuming you spend too much time around computers, and therefore count instinctively in binary).

Note that you would only get n+1 ratios if using the same ratio for each stage.

A normal gearbox uses an approach like this...

Engine -> 1:1 or 1.5:1 or 2:1 or 3:1 or 4:1 or... -> Final Drive

Because each ratio is defined by a single pair of gears.

My proposition is this...

Engine -> 1:1 or 1.2:1 -> 1:1 or 1.5:1 -> 1:1 or 2:1 -> Final Drive

giving ratios of: 3.6:1, 3:1, 2.4:1, 2:1, 1.8:1, 1.5:1, 1.2:1, 1:1

The increased number of ratios available is because each ratio can interact with the other two.

I think the number of ratios offered would be more useful in a commercial vehicle (truck, tractor etc.) than a car. It could be implemented in a 'tiptronic' or auto mode, rather than manual. Hydraulics, or possibly pneumatics would probably be the most appropriate for activating the bands.