Firstly, would different nations have different specialties in regards to the mechs produced? America mass producing Heavies while, I don't know Europe or something, makes lights? Do all mechs have flight capability, or would their be special 'Air Force' models (if they aren't unique, would they simply be able to go to higher altitudes)? Would others be more suited to naval battles (like Oceania which is mostly a collection of islands) and undersea combat?
Also with illegal Mechs. I would assume they would have something special beyond being a mech, otherwise a nation's own military could handle such a thing (or is no one allowed a mech beyond certain specs outside of the UN?)