Well looking at the base game, most of the mods (That aren't grips and sights which are basically all positive), are either part upgrades like the match trigger and tuned mechanism, or modifiers that trade one thing for another, like the extended barrel or the muzzle brake. For one thing, I'd say the match trigger should be laser weapon compatible. Perhaps something similar to the tuned mechanism for lasers? Polished lenses or an upgraded medium would be flat upgrades to damage and/or range. Accuracy is a non-issue, so the equivalent of an extended barrel would probably be worthless. However, an expanded chamber would give the laser more time and room to build up (Assuming my rudimentary googled understanding of lasers is correct), so an increased weight and volume for even higher damage and range would be plausible.
Then we get into slightly more drastic upgrades, an auxiliary heat-sink (Under-barrel?) would increase burst size and reduce firing time (If that's a thing...), but would greatly increase weight and maybe even increase power costs slightly. Next would be the capacitor/power supply/whatever, which would have the options of either being bigger; more damage, more range, less efficient (Damage per UPS charge), or being lighter; the opposite, more efficient, lower damage and range, probably much faster firing. The lens could also be modified, one that spreads the beam and drastically increases damage at the cost of range and armour piercing, while another focuses the beam to do less damage but pierce armor more effectively at greater range, depending on what is needed. There could even be a special high end version that allows you to switch between the two.
Finally, drastic kits like I mentioned would have especially large heat-sinks, lens, etc. To fully specialise weapons towards one particular role but leave them there completely. I'd even like to see a handheld version of the laser gatling CBM.
Personally, I'd like to see all weapon mods craftable, including the gun mods that currently aren't like scopes and tuned mechanisms. It seems stupid to me that I can build a rifle from scraps, even a laser cannon from fairly basic components, but can't grease and file some gun internals or stick some lenses on both ends of a pipe.