I still say of the MMO models/systems I've seen, an EVE Online type would seem to lend itself to a CBT MMO the best.
Much like in EVE, you'd start off in the smallest unit - the Light 'mech. The "crappy" Lights would require a minimum of Light 'mech piloting skill - say, 1 or 2, to be able to use. The higher end ones would require 3, 4 or perhaps even 5. You would have to train each type of weapon system you'd like to be able to use - lasers, autocannon, Gauss Rifles, SRMs, LRMs, MRMs, MMLs, Machine Guns, AMS, etc, etc. Certain weapons would, again, require a minimum level in the appropriate skill.
For those that, for whatever reason, don't want to pilot the big, stompy robots, you could provide Agro/IndustrialMech options. You would use these for your gatherer type characters. The resources gathered would allow production of various trade goods and/or equipment.
Agent types would vary, both in the types of missions they would tend to offer, to who they represent. House, Corporation, you name it, it's there.
You could even port over the "Security" rating from EVE. This would apply whether you were a House troop, or a merc. Fight enough pirates/raiders, you boost your security rating. Do too many raids, steal too much stuff, etc, then your security rating goes down.
You could still include the backdrop of the Inner Sphere and its major events. CCP (the creators/owners of EVE) introduce storyline type backdrops, and there are even important storyline missions that pop up from time to time.