Some ideas for grunt classes (written before I saw egan's last post so some of these were mentioned already):
rifleman
grenadier
heavy weapon
sniper
medic: patches up grunts and mages (and pilots, though probably not as relevant)
combat engineer: repairs mechs and other tech toys (like drones or turrets)
drone operator: get 1/multiple drones to operate. Spotter or support drones probably better for gameplay?
radio operator: stays in contact with the base, can call in arty strikes or care packages or reinforcements?
commander: bonuses to grunt rolls? Like maybe all grunts in range gain a positive trait as long as he's alive. Or just commands npc's?
driver: comes with small vehicle, like a technical or jeep equivalent, maybe higher tiers get better vehicles?
pilot: like driver, but for smaller/weaker mechs (or maybe even aerial vehicles?)
beastmaster: handles (non?)arcane beasts bred/tamed for war.
In terms of battlefields, you could try to combine it with the objective so that there are tactical options for either side. Nothing wrong with an occasional TDM in an empty field like last time (if it's setup correctly), but not every time, you know? You can maybe use the terrain setup to sort of balance out the mission objectives: if one side has a certain advantage like having the initiative to chose the engagement, then making the terrain more advantageous to the other side can help.
For example: "secure that important location", but which is rather in the open. Ok, bad defensive terrain for the defender, so maybe they'll want to relocate. But now what if said location has a thing that the attacker can not destroy (eg. important cultural artifact for mage/radar dish for mechs)? Now hugging the bad position makes it hard for the attacker to attack indiscriminately. So now there are potentially interesting decisions to be made: do defenders stick to close to the Macguffin, or abandon it for better terrain? Do the attackers try to draw out the defenders, or say to hell with it and risk destroying the Macguffin, which will reflect badly on their score? Giving point values is a bit gamey, but is just an abstraction of how well achieving the objectives helps the war effort for either side and lets you tailor the incentives more easily.
For specific examples: forests, mountainous/hilly, swamp, urban (high-rise or more suburban), caves or otherwise underground, in the water (assuming each side can get some sort of underwater gear), snowy areas, savannah, on the side of an active volcano full of environmental hazards. You have a whole planet to work with so whatever the hell you'd like really.