Hit Die: d8
Armor and Weapons: Can't wear armor (natural armor as chain), can't wield weapons not built for Cybertronians (step up damage die for Cybertronian-sized weapons)
Energon-Powered: As a robot, you require 1 Energon fuel per week to run, which is rare and dangerous. Without it, always roll with disadvantage. You begin with 2d3 Energon.
Giant Robot: You're easily twice the size of any mortal. Your unarmed attacks deal d8 damage. You can only wield items custom-made for cybertronians or other giant humanoids. Get disadvantage when you're moving around in spaces built for people smaller than cybertronians. You have minimum 16 STR, and natural armor as chain.
Alt-Mode: As an action, you can turn into a vehicle by consuming 1 Energon. Pick which kind of vehicle at character creation. When in vehicle mode, you can't use items, and you're indistinguishable from a mundane version of that vehicle. Ramming attacks deal half damage to you. Turning back into a robot does not consume Energon. Ammunition is created on transformation and does not carry over.
Car: 120mph, d8 ram damage, 4x encumbrance, 4 passengers
Truck: 60mph, d10 ram damage, 10x encumbrance, 4 passengers plus whoever fits in the trailer
Tank: 40mph, d8 ram damage, 2x encumbrance, 2 passengers, d10 damage cannon with d3 shots
Bike: 200mph, d6 ram damage, 1x encumbrance, 2 passengers
Jet: 400mph flight, 1x encumbrance, 2 passengers, d12 damage missiles with d2 shots
Helicopter: 100mph flight, 3x encumbrance, 4 passengers, d6 damage cannon with d6 shots
Gun: Small enough to be wielded by other characters, d10 damage, 2d6 shots