Some ideas:
Have extremely high thresholds of damage mitigated by atmosphere be converted to air-burst nuclear detonations, without radiation (but perhaps still with dust)? Would cause a bit of destruction to whatever population, structures, PDCs, etc that it would hit.
For balance purposes, we'd be talking an entire 20 damage stopped by atmosphere being the equivalent of a size 1 warhead, for instance, with mitigations below this point being considered inconsequential. To allow medium tech levels who are determined to siege a planet with carronades or lasers, but will need to do it slow and steadily. Might give a much more kinetically satisfying image than just zapping their PDCs down with mesons.
Plasma Carronade Tech: Spinal Mount. Uses the spinal mount from lasers to make your big carronades even bigger, nastier, and more explody at whatever it hits. Essentially would work the same as it does for the lasers, increasing base damage (and max range as a result), with slower reload speed per capacitor capacity.
Plasma Carronade Containment Tech: Works sorta like the wavelength on lasers or velocities on kinetics, except is less effective per tech level (probably just slants the curve a little bit rather than outright increment each damage-level's scope). Each level of this tech increases the size and reload time of the carronade it's applied to.
New Ship Component: Capacitor banks. Function as generators, except the energy they have is 5 times (more or less depending on tech or balance, might be a new tech or an already existing tech in the game) greater than a power plant of the same size, and the power drawn from them is exhausted when used by a weapon. For instance, a weapon that uses 5 power attached to a capacitor with 20 will fire 4 times before becoming exhausted. Capacitors can be recharged in hangar space, but the parent ship needs excess power plant capacity of at least the max charge of the capacitor, and the rate of charge would be determined by capacitor recharge rate tech. Capacitors are also dangerous to use on ships expecting to be hit, I'm guessing that they'd often be HTK 0-1 with internal explosion size equal to half of the stored charge if it detonates within.
Solar Harvesting: A particularly substantially high tech level upgrade of the sorium harvesters, with this addition, stars can sometimes be sources of sorium (rarely), and can be surveyed much like gas giants and will tend to have much lower accessibility, but similarly high stocks if any. Possibility of solar cooling if the star in question is being sucked of all it's sorium?