So a doomsday weapon should have to be big and fixed, unlike turrets. It should have to be built at turret factories or maybe a research lab with lots of expensive and hard to get materials, think the smugglers quest but more hectic. It should definitely be very expensive to produce. It should do high damage and have a long cooldown and a large energy consumption. it should be mounted like a turret and should only point in one direction, forward, like how engines only point backwards. So when you place it, it should have a clear view in front of it (nothing blocking it). To make it less OP, there should probably be a turret control system for doomsday weapons, +1 doomsday weapon (legendary). Doomsday fragments can be found and combined at a research station to get this legendary upgrade. Bonuses should apply like for normal turrets.
Doomsday Weapon Types: (open to suggestions, will place them up here but don't get too wild)
Magnetic Accelerator Cannon - launches big metal slugs, huge damage, high range
Laser Death Ray - big laser that lasts until energy is depleted, high damage overtime, short range
Big Nuclear Missile - slow moving, seeking, huge damage, medium range