So, I could talk specific numbers but it's likely the dev team has access to more variables than I do, so I'll talk mostly in generailties:
1) Aliens should shoot more
Aliens currently spend more time moving from cover to cover than they do shooting back. This is true of Sebillians, and especially true of Brutes. If X2 aliens have a minimum -to-hit like thy did in X1, then it seems like the aliens in X2 spend too much time trying to get to the point where they can hit a target. I don't know quite how you'd address this, as aliens in x2 have the same problem as they did before the minimu to-hit was introduced in x1. When an alien in X2 can hit a target, they stay where they are and spend all their TU shooting. Sometimes this is a good thing, but most of the time it makes them a sitting duck.
2) Brutes needs more dakka
Sebillian Brutes are not scary. They don't fire their LMGs, they are too easy to suppress. I would suggest either doubling or tripling their current bravery to make them much harder to suppress, because once you suppress a Brute they sit their sucking their two thumbs. Even warriors have more fight than Brutes! They're supposed to be RAWR, instead they're roar . Also, somehow, force them to fire their LMGs more. This may be not how you picture it, but the Brute has some parallells to the humble Ork. Big, resilient, dumb, always up for a fight. I picture a Brute just blazing away and when it gets up close using its LMG as a club.
3) Human ballistics could stand to be nerfed a little re. damage
After some interesting revelations from Solver, I experimented with ballistics a lot. The curent damage output from human weapons is a touch too high to make aliens fearsome. I've experimented with rifles doing 20pts damage per bullet, shotguns do 18 per pellet, LMGs doing 20 (the extra bullets per burst tend to multiply quickly), snipers doing 30.
4) Sub machinguns and pistols could stand some reworking
As things stand, I don't currently need either pistols or submachinuns. The basic primary armament does the job, and by the time I get to shields I've usually got access to either laser or converted pistols. istols ad sub machineguns need to up their gam to be more competitie. I would suggest making pistols really cheap with their shots. That's not how actual pistols are, but it's a field they could compete in. Sub machinguns, I'm not certain about. Cheap bursts?
5) Give us a reason to manage ammo with human ballistics
Solver is right - you never run through a magazine or belt with the current game. I've tried a couple of things - nerfing ballsitic damage and making it hella cheaper to shoot boolet but I'm inclined to agree that if there's going to be 10 soldiers then the magazine sizes need to drop a little. Perhaps 15 shots for the rifle, and 40 shots for a LMG belt. The two weapons I've come closest or actually had to reload have been the sniper and the shotgun.
5) Give lasers infinite ammo.
I talk about this every balance thread, but I sincerely believe that setting the regenerate value the same as the cell size is the most unique thing you could do with lasers. I'd suggest with the laser sniper you give it a unique magazine with 1 shot (I've tried making this myself, but I'm missing something I make it both in the strategy and the ground combat folders, but I can't quite make it work - what am I missing?), making in effect a bolt-action weapon but seeing as it regenerates 1 shot each turn and each shot is very expensive that just makes it in-line with other lasers. I'd also suggest setting the laser LMG cell size to 5 shots. Not having to carry ammo and lightness could be the unique selling point of lasers, just like it was back in the Xcom days.
6) Rework Converted weapons TU usage and introduce other drawbacks
The TU usage of converted weapons is currently too high. It makes them severely unattractive compared to ballistics, and you don't really need them until you run across Androns. Converted weapons could be an excellent bridge weapon type towards human equipment, and be the harbinger of gauss and plasma. I'd suggest introducing degeneration with the alien plasma weapon, so the 16-shot cell starts ticking down, and possibly as Solver suggests, make ammo limited and dependent on aliens. It might be an idea to make Cmag weapons heavier, to reflect that they are carried by the burly Sebillians and possibly make the Mag LMG so heavy that it can't be carried by a unassisted human into combat.