For my own weapon ideas, I give you these:
Tri-Barrel Shotgun: (This does exist in reality, I think.) Very much a unique weapon. Instead of having x3 modifier, it has a nicely increased x4 or x5 modifier. However, it also takes 1.5x the time to reload as the regular Double Shotgun.
Mining Blast Cannon: (Unique Weapon.) Uses Energy Cells, when fired, sends a short-ranged yet extremely powerful energy shockwave in kind of this pattern. The rationale I used for this kind of weapon to be in a UAC base is simple: They had to have some kind of effective mining operation going on, or else they'd be completely helpless without transports.
X
@X X
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The Shovel: (Unique Weapon) Oh, now you'll really get going. This weapon hits for less damage than the chainsaw, more than the Combat Knife, but when you hit with it, the enemy is stunned for 2 turns. I know of nothing that wouldn't be stunned by being smacked on the head with a shovel. Could inspire "Angel of Shovel" challenge, where you must beat the game ONLY using a shovel and a Nuke at the end.
Flamethrower: (Unique or not, Developer's Choice.) To make the flamethrower easier, I hope, to code, it will have a range of 5 squares, and go in a straight line. Each shot with the flamethrower hits for the same amount of damage as Lava, and it takes Energy Cells to operate. It fires very quickly, obviously, since it's a continuous stream. Eats cells like candy.
Mancubus Arm: (Unique) Fires 3 Mancubus Fireballs, with their respective advantages and disadvantages. Eats rockets like candy.