In my experience, the Heavy Plasma is too close to an ultimate weapon. Any solider can carry it, and 35 shoots for clip allow full auto fire during most of the mission.
This Mod makes it, and all other plasma weapons far heavier, with ammo clips that are also heavier and hold less ammo. Weak soldiers (strength below 30-35) will probably be better carrying Plasma Rifles. And the limited ammo makes you think before firing a full auto burst. This can also make the Plasma Rifle a better choice in some scenarios, because of its larger ammo clip.
Extra challenge if you use the 80 item limit, as you may not be able to carry extra ammo clips for everyone
Some alien races have been made a bit stronger to compensate, and in some missions they will carry extra ammo clips to compensate.
Heavy Plasma: Weight from 8 to 12 - Clip : Weight from 3 to 5; Ammo from 35 to 6
Plasma Rifle: Weight from 5 to 9 - Clip : Weight from 3 to 4; Ammo from 28 to 12
Plasma Pistol: Weight from 3 to 4- Clip : Weight unchanged, still 3; Ammo from 26 to 10
I've been playing with this mod enabled for a while, with no apparent problems. However, as this is my first mod, I may have overlooked something, I appreciate any feedback you may have.
(Extra note: I was somewhat surprised to find that new soldiers are 75% male, 25% female. This mod changes that to 50%-50%)
To install the mod, simply extract to your mods folder (usually in the OpenXcom folder in your Documents folder for Windows)
Made the Heavy and Rifle slightly lighter (14 to 10 and 10 to 9), and increased the clips capacity (5 to 6 and 8 to 12)
Also, in the original game, the Heavy Plasma Clips take a lot of space in the General Stores (more than the Heavy Plasma itself, and 3 times more than any other clip.
So I made the clips smaller, and the HP larger (about the same size than other items that take 3x2 in the soldier's inventory)