Multiple level signal transmission
There are some good reasons not to do what you imagine.

First, the signal level at your receiver will vary quite a bit depending on the properties and length of your cable (and if you are out of luck, also on the phase of the moon). You will have to calibrate your receiver for the particular cable.

Reflections of large signals (0->1111 transition) might swamp small signals (0->0001 transition).

A 4-bit (=16 level) flash AD at 100 Mhz is not trivial. 

Generally, doubling the baudrate is often much easier than doubling the number of levels that have to be resolved.