To start I think there should just be one basic 4x4 vehicle like the one in Firefall but with 4 seats and a gunner slot.
Aesthetics: To help with game upkeep and development costs there should be a lot of customization such as paint colors and patterns , hood ornaments, horn sounds, engine sounds, headlight styles, roll bars, chasis types, rims, etc. Basically alot of aesthetic options for people to spend money on to make a unique EM8er Crawler.
Functional Modularity: Aside from monetized modularity there should also be functional modularity. This being different weapons you can mount on it, or maybe one has like an ammo replenisher while another one can heal people. Earn engine upgrades to increase speed. add melee spikes and shredders to run down enemies for the joy of running them over and watering the ground with their blood. Even sensors that spot cloaked enemies or increase sensor range. Basically make the vehicles have many different exchangeable attributes to make them unique to you and have some limited use on the battelfield while still stressing the use and mobility of your suits. It would be cool to see a better more customizeable version of the ranger that everyone could make their own and even name.