High tech
Laser Gatling
Replace chaingun
Tesla gun
Replaces flamethrower for controlling little creepy-crawlies
Plasma grenades
Low Tech
Nailgun
A gun that shoots heavy spikes that slow aliens. The larger the alien the faster the slowing effect wears off. A large alien would only be mildly inconvenienced by the bolts. A small alien with slow to crawl, even pinned to walls (please yes!). The trick to balancing it would be the rate of fire on the bolter, the duration of slowing effect and damage done. This would be a killer weapon when used in a fire team but not so strong 1 on 1. High damage/slow rate of fire and you get a viable sniperrifle, low damage/high rate of fire and you get a funky machinegun
Golf club
This would be a random item you can pick up. 1 per map.
Thermal Lance
It'll get you into a bank vault and it'll take down a few acid tubes.
Demolition pack/ dynamite
Like a grenade, only improvised.
Pressurewasher flamethrower.
Different skin for the flamethrower. No self-respecting colonist has a flamethrower hanging around but he might have a pressurewasher. :)
Some of the old tremulous weapons as randomly spawned loot around the map.