While I have mentioned this to my brother who also plays the game, adding vehicles is... quite difficult with the current balance in the game.
If you gave me 4 Mechs and 4 Vehicles (say) for a game, those 4 Vehicles would almost certainly be LRM Carriers and my Mechs are the spotters/brawlers who both spot for the LRMs and also are tasked with preventing the OpFor from getting in range of those vehicles.
More generally, the biggest difficulty with expanding the number of combatants on the battlefield is that the battles will attrit more. If you pit a company against another company, with current rules, I don't see how you're not losing a Mech or two every combat.