If you want to look at upgrades, look at Gooncode's limbs and various upgrades for inspiration. Light/Standard/Sturdy/Heavy limbs, with accompanying mechanical differences in Speed vs Durability, as well as Treads. Consumable repair/recharge packs, a rebooting upgrade that uses power to get out of stuns, HUDs, shields against damage types, even a teleporter letting them go between the beacons.
Customization is fun, encourage it!