Personally I'd roll with the "engine heat sinks (minimum 10) are SHS, any additional can be freezers" idea. This way a Stinger, for example, can have its base ten heat dissipation, and so can anything up to a 250-class engine. The classic 300 series engines would have 12 SHS in the engine, none of which can be freezers, but you still pay the two tons to fit the two SHS in the engine. The 400 series would be six tons of SHS in the engine before you get to doubles.
It'd penalize heavies, I think, since you can't get freezers until you pass the engine HS, but it'd give lights and mediums a bit better heat dissipation. It'd work well enough with the small, compact size of energy weapons, and you can't mount a lot of DHS, but it does save you some hard tonnage you can use for things that don't take up space (looking at you, armor).
Alternately, there's 'Go back to the old rules where you account for ALL heat sinks' which would dramatically change the survivability of various 'Mechs. Thinking of the Marauder, Crusader, etc; stuff with empty torsos that suddenly find themselves at least somewhat critpacked.
Let's see how the former works, for the TDR-7M. It gets 10 free engine sinks, all of which would be SHS. It comes with 15 doubles normally, so we'll make the last 5 Freezers, all of which need to be allocated. Fortunately it already does that; the only change is that it drops from 30 heat dissipation to 20 (10+(5*2)) which isn't that bad - it only builds up 29 heat anyway on a full running alpha-strike.
A Battlemaster, meanwhile...let's take the Royal Beemer. 13 free engine sinks, so it's paying 3 tons for 13 SHS total. The other four HS would all be doubles, which again are allocated, but chops the heat dissipation back to 21 (13+(4*2)) and really forces a rethink of the design. I mean, two ERPPCs and an LPL for 21 heat dissipation is pretty rough...
Maybe as an alternative, the fusion engine has ten points of its own cooling system - the FECS - and ANY actual heat sinks added can be double or single, but all must be paid for. Think of it like this. The engine naturally produces heat, but is shielded as in basic canon. If the shielding is damaged once (engine crit), you leak 5 points of heat into the system, which is immediately taken up by the FECS' 10 point heat capacity. A second hit to the engine (more specifically, the engine shielding) means a severely exposed engine core, leaking 10 points of heat - just enough for the FECS to maintain cooling and keep it from going runaway, though movement or doing anything other than shutting down ASAP is going to build up heat the FECS can't shed. Punch in ten FECS slots into every 'Mech, ostensibly in the three torso locations if possible?
That'd turn the Battlemaster above into a bit of a tizzy, since it's buying 7 Freezers on top of its ten FECS points, so it's getting 24 heat dissipation (10+(7*2)) which is a little better but it might still want to rethink that gunload. And it's got the empty crits to do it, at the moment...unless you make the FECS ten heat sinks required as well.
I'm just shotgunning ideas, mixing them together to see how they do. Stepping back entirely and trying something different, what if we go with XL HS? Apply all Standard and XL heat sinks, only XLHS are 0.5 tons and 1 crit each, still shedding only 1 point of heat. Thermal efficiency only goes so far; you can make the heat sink lighter but not smaller or more effective. At least you could afford to slot a lot more of them than the three-slot DHS, and XLHS would fit nicely in the CT and legs as well. As a bonus, you have to slot ALL heat sinks, so you're still limited to a rough max of mid-20s heat dissipation assuming most slots in the torso, legs, and a few arm slots are taken up. It'd give back some survivability, I think, since you're filling in slots.
I kinda like that last idea. Time to make a mech or three...Thunderbolt, Crusader, and Marauder, the former because I love 'em and the latter because they're infamously terrible torso bombs.