Here is what I'd set for MWO.
First apply these effect to all mechs, if they hit that heat in realtime (Percentages can be adjusted based on what is best for applying the effects, so instant application could see lower Percentages, a delayed application of effects should have an appropriate chance to apply):

(Since, there are no effects below 5, 4 to 0 is not shown in chart)
Next I will cut Heat Sink Capacity for both SHS and DHS to 0.1. So this way, any mech will get 30 capacity, plus their Heat Sink count for Capacity.
What this does is severely limit the heat spikes we can allow on a mech. So the capacity difference going from 10 HS to 40 HS is 3 (each group of 10 HS, gives 1).
So we can still fire a single ERPPC and any combination of weapons up to a spike of 17 when stationary on a cold map, and the first penalty is only an Override-able Shutdown at 14 heat.
So, if a player chooses to fire a combo of weapons above that threshold, they will shutdown (if they did not Override first) and would incur any appropriate Heat Effect (the first dangerous effect begins at 18 heat).
So as in the original, where each weapon rolled separately, we would need to fire high heat weapons separately, while certain low heat weapons can still be fired together up to a range of a ~14 to 17 point heat spike.
Heat Scale penalties could still be adjusted, but this is going by weapons' base heat values first anyway, and I mock up another table of various weapon combos too if asked.
And if any mech is negatively affected, mechs can receive quirks to aid said mech on a case by case basis.
And something that I've read about with Clan mechs is that they carry redundant weapons to defend against through armor criticals (something MWO doesn't have) that can take out equipment. So they really didn't carry all of that weaponry to group them together to fire them all at once, as often as the can as we currently can in MWO.
Here is a worksheet that shows how that could look in regards to heat spikes, the bottom portion shows the amount of heat dissipated over a 10 second period:
With this in place we will receive appropriate Heat Effects according to how much heat we gain, and we can look into having all DHS being true .2 dissipation instead of having a difference between Internal and External DHS.
Also, we can utlizie the ability to add quirks where mechs would need a boost, such as energy heavy mechs for example. The Awesomes where the first get a valuable quirk after all, I'm sure more will follow on a case by case basis.