Pistols (Blasters?): super inaccurate from max range (8 tiles) but gains immense accuracy as you approach melee range. Another idea in the same format is to make the damage increase instead of accuracy.
Carbines: no inherent critical strike bonus, but there's a chance to volley every attack. Potential chances: 50% chance to shoot one extra bolt, 25% chance to shoot two extra bolts.
Rifles: the "built-in" power of the ranged weapons. Rifles have decent accuracy and damage, but have a difficult time landing shots on very close targets.
I still need ideas for the melee setup, but I'm very happy with the general idea of the new ranged mods.
Here's how it ended up working:
Pistols: they all come with a nice accuracy bonus built-in. Your accuracy is decreased for each point of distance between you and your target. You get a helpful message informing you whenever the accuracy penalty exceeds 20. I'll probably put in an option to disable the "helpful message".
Carbines: overall works as I described originally. They all come with very low/negative accuracy and little to no critical strike, but a full volley is quite devastating all the same.
Rifles: they all come with a hefty accuracy penalty. Your accuracy is increased for each point of distance between you and your target. You get a helpful message informing you whenever accuracy bonus is less than 10. I'll probably put in an option to disable the "helpful message". This seems familiar.