How about a System where you get more CBills if you are playing a Mech that nobody else wants to Play?
For example if you Play a Jenner K instead of a Cataphract 3D, which is used 3Times as much, you get 3 Times the Reward (Sounds much, but after some Time there simply wont be any more Mechs that are played so much more then others, because CBill-Farmers will farm the Hell out of low used Variants. The Bonus could even be way smaller and there would still be enough People who would get hooked on it.). There should be enough Statistics to determine the needed amount and adjust it automatically with each Update.
We would get more diversity like this. Maybe even Tonnage Limits would be redundant.
Smaller Version could be to reward smaller Mechs with more C-Bills for the same damge output which would solve Problems that will come when Tonnage Limits show up in the Game like long waiting times if your are driving an Assault (Personally i dont like this Option as much as the first one because some lights for exapmle are still much better then mediums or even heavies in Damage output....Jenner F for example).