Include armor penetration system which is usually used in tank sims. Internal systems which get damaged depending on what kind of ammo hit, how much kinetic energy it had etc... Think of it as a tank sim, but vertical, and with legs.
Make it so the Mech can fall down if it turns too fast with too high speed. How many crew members do you wanna have in a mech? Make it so the pilots are vulnerable.
What kind of weapons do you wanna use? Handheld weapons? If yes, make arms, legs etc cutable. How mobile will be your mechs? Are they aerodynamic?
Put in stuff like ejection seat, screen wiper, startup procedure much like the A10c in DCS, complex electronic system, which can fail in case of damage, making you a sitting duck.
Maybe being able to jump out, grab a sniper, shoot someone else out of their cockpit and then taking over the mech to continue fighting?