Although I don't own a 3dgs version anymore I'd like to add my two pennies:
a) Use a script language that only provides some information about the environment (the more you know the more complex your AI can be) and to control rotors, motors, weapons of your robot. That way it's almost impossible to cheat because the mechanical forces are calculated by the physics engine.
b) A credits system where you can spend your credits on certain parts of your robot, i.e. there's a chassis to which you can plug several extensions. These extension cost credit points. You can then build different robots like fast ones which can't heavy weapons (because then they wouldn' be fast anymore), robots with heavy weapons but less armor (no credits left to spend) etc.
c) add some basic functions to the scripting language that makes building your robot easier for beginners like "rotateTurretToEnemy" etc
*Weapons are limited in ammo, have different recoil times
*Chassis (and maybe also the extensions) should be customizable (change skin)