You'll always get people who want to just jump in, "drive a big robot and blow some stuff up", but if you want a die hard community that will come back, month after month, you need to have the game set up so that joining a faction means something more than a different paint jobs and chat channels.
There needs to be battles over planets, or even locations on planets for resources, factories, etc. Read any Battletech novel and just about all of them revolve around epic battles across planets. If a faction wins a battle, they should have access to those resources/factories until someone else kicks them out. What's the point of a 'Universe at war' if the planets all reset once a week to defaults or there's no realistic rewards/consequences for attacking/defending locations? If that's the case, you'll see the player base drop off drastically after the initial few months rush into the game from long time BT fans.
Mekwars was a stab by some folks to do something along these lines using the Megmek client and it might be worth looking into.
So the big problem with this rears it's head when one faction has a lot more players than another and just swamps them. Well, a fix is that each location has a set amount of defenders and attackers that are allowed. If there's not enough players to fill out the slots on either side, then an AI handles the remaining units. If there's too many people only, then oh well, only the set amount of slots can be played by players, and everyone else has to wait for the next battle.
This has two benefits. First, it stops any one side from just swamping other side due to imbalances in faction populations. Second, it will have an indirect effect of balancing populations out across the factions, since if you're in a faction you always have to wait to get into battles, you're going to be looking to move to another faction pretty quick.
You could easily have a great crossover Real Time Strategy (RTS) and First Person Shooter (FPS) hit on your hands if you do this right.
Edited by Meatball, 01 November 2011 - 10:33 AM.