3: this can be applyed without bars, by making the ship (or the frontal part where are the cannons) become gradually red.
I'd like it becouse adds a lot of strategy (evade, plan the attack according to enemy positition etc), instead of holding shoot all the time without a reason to stop it (quite boring! :P).
This feature has no need to be invasive or ruin the frenzy: the max overheat could be reached after 10 secs, where the cannon can always shoot 5 bullets per second, and a full refresh can take 2 secs.