I was think about that before and have an idea.
First off, the goal is make the combat vehicle with a fusion engine, as the viable choice as much as a battlemech. I don't care for lesser ICE/Fuel Cell vehicles, for they are just the chipshots, as same as the lesser Retromechs. But fusion engine is different. As proper battlemech uses fusion engine, it should be good as much as on the vehicle as well.
What I aim is, to make combat vehicle that is surely win against the battlemech with same tonnage, if the battlefield have no terrain feature and battlemech is not engage the CV in melee. Thus, in the real games, battlemechs can abuse the terrain feature and/or melee attack to beat the vehicles(in conclusion vehicles are still not so good against battlemech on the real games), while vehicle is not the only 'lesser' asset as long as you pay for a battlemech-level of techs and gears.
The idea is, to Combat Vehicle with fusion engine(of all types, such as XL and XXL);
-Change the space to 9+(tonnage÷5), instead of 5+(tonnage÷5). So a 22 ton VTOL may carry up to 13(9+[22÷5]= 13.4->13) items of equipments, instead of 9(9+[22÷5]= 9.4->9). While a 45 ton hovertank may carry up to 18(9+[45÷5]= 18) items of equipments, instead of 14(5+[45÷5]= 14).
-Change the weight of internal structure by 5 percent(10 percent if superheavy) of the vehicle's tonnage, instead of 10 percent, rounded up to the nearest half-ton. For example, the weight of the internal structure of a 45-ton Combat Vehicle is 2.5, instead of 4.5.
-Turret have no extra tonnage; items mounted on the turret adds no further weight at all. For weapons does not mounted on the turret, see 'quality of life change for tank destroyer' below.
-Fusion engine's weight is same as battlemech; it does not gets +50% increased weight.
-Weight-Free Heat sinks is 20, rather than 10.
-Armored Motive System(283, TO), Limited Amphibious(302, TO) and Environmental Sealing(303, TO) adds no weight at all.
-Fully Amphibious costs 1 ton per 25 tons of total unit weight(rounded up to the nearest 0.5 tons).
-Automatically install Combat Vehicle Escape Pod(309, TO) for free.
Well, I don't think that these change makes the vehicle overwhelmingly powerful, and I don't intend that either. But these change makes it powerful as much as the battlemech... perhaps.
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And... for so called 'tank destroyer', that mounts the weapon on the body sections rather than turret, it is usually not worth at all on the construction rules I think. Although making turret makes more parts and makes the more weak spot for this....
I think that all combat vehicles are deserve this treatment, fusion or not.
All Combat Vehicle are have these tweaks;
-All weapons not mounted on the turret gains Accurate Weapon Design Quirk.
-Choose either Front or Rear when construct the Combat Vehicle unit. Reduce the weight of all weapons with 1 tons or heavier on the chosen location by 10%. If apply the 'better fusion CV' above, CVs with fusion engine group reduces 20% of weight instead of 10%, since they don't pay for +10% weight when mount the items on the turret.
Fluffwise? Because they are the vehicle, when mounted on the center of the vehicle it surely stabilized and it requires lesser components to be functional. Since weapons on the turret gains no benefit from this, I don't think that it is required to the unit must not have a turret location to enjoy this. If you need Demolisher II-X, then so be it.
So, it will give you the choice, either put the weapon on the turret, put the weapon on the body to save weight, or mount even bigger weapon on the body instead. But is -10% or -20%(for fusion only) enough reason to forgo turret, or it needs more discount?
The reason of allow to pick either front or rear is because of gun trailer(heavy gauss); Its heavy gauss rifle is aim for the rear, instead of front.