It is more a global balance problem, no ? in vanilla's, the game has a old school progression which began with useless obsolete weapons, and goes toward "grosbill" has we say in french : supersoldier, superarmor, superweapon (laser rifle with no ammo , good accuracy, light weight and autoshot, or heavy plasma with nearly no penalty). This is that the problem.
In addition, vanilla's aliens don't really use armor and damage modifiers then laser rifle and heavy plasma are good against everthing. (and IC weapon are nearly useless except for the light)
For better balance, we need already a compensation to a great power. For exemple : heavy plasma could be powerfull and accurate, but not also light and clip with many ammo. You can make power+accuracy, but weight+limited ammo. Laser rifle, can't be light, unlimited clip, accurate and powerfull. It's too much we need make some penalty.
I work personnaly on a balancing mod where I have made some choices :
- laser still unlimited ammo, but are really heavy. bests soldier can't carry many other thing with a heavy laser. weakest can't even carry one. Laser rifle are 2x weightest than firearms. And I have remove all autoshot mode for laser weapon (always think ridiculous, if who want more power with a laser, just make it continuous).
- plasma still powerfull+autoshot, but loose accuracy. many. (it make room for new aliens accurate weapon, like sniper plasma etc.)
- modifications in armor and damage modifiers of all aliens, to make some resistant to laser, but not to plasma, or AP. And some the opposite. In general, aliens master have opposite resistance to their terrorist, so, player need various weaponry to win. Some of them are resistant to laser AND plasma, so even in the end of the game, you'll need some AP or HE weapons.
Add new tech or weapon without rebalance the game don't solve the problem that many item are useless because obsolete too quickly. It is just like a delaying research no ? why don't just make the current laser research longer ?