A reason to have a con score between 3 and 9: you can have 1 active drug status (jumping bean, adrenaline, morphine, etc) for each point of con, more than that and you suffer a debuff of some sort (sick, over-medicated, etc) or maybe you just can't take more drugs
if you want to be harsh, make it 1 status per 2 points of con
Also, you could increase benefical drug durations for higher con characters and increase drug cool-downs for lower con characters (and lengthen debuffs, like adrenaline fatigue).
It bugs me that you can't jump down to or up from rails to the platforms on either side. Could be an agility check, but everywhere along the platforms.
I think it would be fitting if strength lowered your effective armor penalty a teeny bit. People might take higher strength to reach certain thresholds (like 15% for nimble) with a given component combination. It also gives people more reason to increase strength besides melee damage and metal armor (and countering crippling strike in metal armor). Carry weight is there, but I don't think anybody is increasing strength to increase carry capacity.
Kevlar (as it is) and antithermic should have their armor penalties switched. Antithermic is VERY good for just +5% assuming you aren't a stealth build or aren't currently concerned with stealth, whereas kevlar is awful on leather armor and decent on tacvests, being "good" only on very heavy tacvests. This also kinda makes blast more appealing since if you're going 10% for heat protection you might instead go 15% for explosion protection in general.
I think it would be fitting if tungsten knives required 4 strength, and .44 hammerers require 4 or 5. For the knives I just think it makes sense; there's no real balance concern considering that tichrome knives are probably superior (however there is the crippling strike problem; it would be fair to require 3 str for regular knives). For pistols, there's little reason to use anything but a .44 at the moment; requiring 4-5 str might motivate people to use lighter calibers on their gunslingers, and for those who do take 5 to use hammerers, well now they qualify for steadfast aim at least.
I think it would be interesting if robots came in different metal types and had their stats change to reflect it (and their appearance would indicate what metal they're made from). Steel would be the most common, but tungsten robots would be slower and better-armored, whereas tichrome robots would be faster and less-armored (but have better heat/energy resist). Likewise, robots ought to sometimes drop corresponding metal plates when they "die", with stronger robots dropping better qualities.
You've said before that you want to do something about energy shields; what if they affected the outgoing damage on your ranged (including psi) attacks? and maybe lowered your throwing accuracy? You can still just wait until the end of your turn to activate your shield, but you can't do that on subsequent turns.
Maybe active energy shields just lower your effective guns/xbow/throwing/psi skills based on how much they protect.
A big change that would help balance the game (and give it much more tactical depth) is an overwatch/readied-actions system. It would certainly help close the gap between fast builds and tanky builds.
Tying in to that suggestion: what if grenades could be bounced off walls/around corners? perhaps with a fuse delay? This would help deal with overwatching enemies and give characters some more area denial options (at least against intelligent enemies), and of course it would be used against the player as well. It would also give you a way of "nerfing" grenades by making it much harder to bounce/time grenades with lower throwing skill, in addition to general placement accuracy. As it stands, you throw grenades at clumped enemies almost like a mage fireball instead of something resembling their real-life use; irl you don't see a group of enemies in the open and throw a nade, you shoot them; grenades are mostly for enemies around corners or behind walls. Also, if you "cook" a grenade by activating it with the intention of throwing it on your next turn, but you get stunned or incap'd before then...
I think item qualities should be lowered across the board by about a third, at all stages of the game. q160 should be max-tier end-game quality.
TM needs the nerfbat and everybody knows it. Really it's just LTI, contraction, and stasis. LTI could be a simple fix, like increasing it's duration cooldown by 1 or 2. Contraction is just too good, even with it's occasional drawback; I made a thread a long time ago about how I think it should be a feat that lets you cast dilation on yourself for the opposite effect and requires 7 will; if not that then do something more than it's current drawback at least (perhaps it's drawback always occurs, but is weaker with higher skill). Stasis is strong, but what makes it OP is it extends beneficial durations by another round (on top of basically letting you have two turns in a row and wasting enemy buff durations); if it just didn't do that, if your buff durations didn't extend into your free turn, it would be noticeably weaker (but still very strong).
However, people also say that you can handle anything in the game with distortion/ripple. I don't have any ideas on what to do about that, if anything.
Lastly, and this isn't a suggestion, it's just an idea: I drive forklifts at my job, and seeing all the unusable forklifts in the game has always kinda bothered me. What if there was a spot in a dungeon that is blocked by big heavy stuff (like stacked crates) that could be moved with a high strength check, or you could repair a nearby broken-down forklift with a mechanics/electronics check and operate it to get rid of the obstruction? I know the crane in the waterway facility is like this, but still I'd like to see it.