I'm going to talk about a few different things in this post:
1. grenade launchers
2. heavy guns
3. energy rifles
4. weapon damage in general
You've confirmed energy rifles for infusion, and mentioned adding a fusion cannon, so I assume you will do the heavy duty weapons as well. I know infusion will have different mechanics, but I still want to go over what is wrong with them in underrail 1 so we're on the same page.
To start with, grenade launchers are turbo busted. Regular grenades are already strong, but with nade launchers we can shoot multiple per turn with no CD, and with scaling damage and critical hits. Three-pointer let us have critical hit explosions, and I know Nagas can crit with their blasts, but letting NLs crit like any other weapon seems ridiculous, and scaling damage doesn't seem right either. Just letting people shoot multiple nades per turn is already pretty strong, so I suggest having a set damage range for a particular kind of nade (like with regular grenades), and the characters skill merely determining accuracy; this means the components of a grenade launcher don't matter much but that's a lot smaller problem than letting them be so busted. If you insist on them being able to crit, just give them an equivalent of three-pointer. To end this point: Eidein has a video of him killing the arena masters sewer boss fight with thermobaric nades; those guys are in blast suits, have impossible evasion, and boss hp and he still managed it. That ought to be impossible.
Next, heavy guns. The main problem here is that ARs are already strong enough for anything in the game (and frankly are pretty overkill already), so there's just no reason to have an even stronger weapon and then an EVEN STRONGER weapon beyond that (if there was just ONE minigun, a unique, that'd be fine, but you have four frames/two calibers/three engines; all for a weapon that will virtually never get to shine). With the possible exception of the compound, theres just nothing that justifies it. Tchort on dom can be killed with a single AR burst and he's by far the tankiest enemy in the game. So either you'd need to add even bigger and harder fights (which you've already said the game has too-big fights already; too many enemies acting per turn which slows the game down too much) or you'd need to lower player damage overall so that the heavy guns take up ARs old place.
Energy rifles: kinda the same problem here; if I can 1hko pretty much any enemy with a plasma pistol, what do I need a plasma rifle for? I once had the idea that energy rifles could give us line attacks like plasma beam, but I get the feeling that's not what you intend.
So I think the proper course is to rebalance player damage overall. ARs should be the moderate-but-leaning-towards-"heavy" (and also longer range; a fight against multiple enemies in melee should be practically impossible for an AR imo) choice for the middle of the road soldier, which means you should reallllly struggle to handle the tankiest enemies and biggest fights with an AR. In underrail 1, you can easily, I repeat EASILY kill nagas or tchort or whoever with a lowly knife; I've seen a video of sheepherder killing a naga with a shiv. IMO that should be impossible, and to beat one with a knife at all I think you should need an optimized experienced build, a high-end knife, and every edge you can get (blasting cap, corrosive acid, EW-stacking, multiple damage bonus sources, etc). The weapons that feel in the right place to me are hammers, spears, and sniper rifles which btw most people consider to be the worst weapons (outside of metal gloves and crowbars). Swords are close to being right but I know firsthand how easily you can buff up and chop down even the big robot enemies without much trouble if you know what you're doing.
People will complain: "If I can't kill a naga with a knife, why would I ever play a knife build?", but knives ought to be a weapon choice for characters that aren't generally too combat oriented; your thief type characters for instance, characters who can stealth/hack/run/whatever past the really tough enemies. Whereas your LMG build isn't generally going to be some ninja or james bond infiltrator type character. Another factor is when a given weapon type becomes available; perhaps instead of "minigun builds", a heavy ranged tank character just upgrades throughout the game from (starting gun type) > assault rifle > lmg > minigun.
What would justify a lmg over an AR? What would justify a minigun over an lmg? What would justify a really optimized minigun build? or a plasma rifle.
Picture attached: a solo (no aegis or turrets present) DOM beach defense with a single non-crafted 5mm AR (no gun lube). Just stand still and burst.
