The mecha parts would be broken up by some FPS/TPS sections, where you're playing one of the civilians/rebels and mostly trying to survive the mayhem the stompy robots are causing. A neat part would be that shit you did in one mode would in some way or form affect the other mode in a big or small way.
Something like >knock over/blow up setpiece to stall enemy or don't because it's full of Valuable Commodity
There's an on-foot section to happen right there not long after, it's pretty standard if you didn't knock it over, but it's torn up, cover is sporadic, and visibility is low if you did?
I kinda want a FPS where you have to choose more than just gun-attachments-perks, but also things such as belts/pouches/packs, armor plates, and possibly even character physical build. You could forsake armor for a few leg pouches and an SMG, and be able to move across the battlefield with not only high speed, but high technicality/ability (high leaps, parkour, etc.). The lack of armor means that -all- weapons are going to deal lots of damage no matter where they hit.
Health doesn't regen, though some perks can add some functionality to the health bar. Some units may recover up to 30/100HP, others take less damage after the first time they take damage (adrenaline), etc.
Body type changes weight tolerance, movement options, hitboxes, jump height, even total health.
A system to track pain alters aiming, screen shake, brightness, movement speed, etc: even if you carry an advanced medkit capable of stabilizing injuries taken and recover a small amount of HP, a separate painkiller would be needed to ease the difficulty and shakiness of aiming, stilted/wobbly or slow movement, and/or tunnel vision.