Back in 2nd edition, Plasma weapons could fire only every other turn, as the Imperium used extra failsafes on their cooling mechanisms. Chaos Space Marines cared not for such things as safety in battle and went the opposite route, overcharging their plasma weapons. This meant they could fire plasma every turn, but they had Gets Hot! Quite literally, overheating plasma was a special rule for Chaos.
"Given the contempt held for life by Chaos Space Marines it's unlikely they would be willing to trade off less firepower for a safer weapon anyway."
The rule proposal I have is as follows:
Any plasma weapons (including Plasma Pistols, Plasma Cannons, Ectoplasma Cannons, and Combiplasma) fired by a unit from Codex: Chaos Space Marines (or any related faction) replace Gets Hot with Gets Really Hot. A weapon with Gets Really Hot fires an additional shot more than would normally be allowed, but each roll of 1 means two saves must be taken/two HP are potentially lost.
For Rapid-fire weapons, this means firing two shots at maximum range, three shots at half-range (like First Rank Fire, Second Rank Fire for Guard/Lasguns).
For a Helbrute under the effects of Fire Frenzy, this means the Cannon fires four times (it fires an additional shot, twice).
Let the lackeys of the False Emperor have their coolant systems and emergency overrides. It's not like they work after all...