The three unused, original weapon sprites - laser, Sheep-on-a-rope and black gun - would be great additions of new weapons with unique mechanics.

Add them as default weapons.

My proposals on the weapons' mechanics:
1) Laser:
a two-shot weapon with low damage (~15hp);
fires a laser beam for a certain amount of time (~10sec);
can be aimed during firing (like the flamethrower);
the beam will cut through terrain and create a tunnel too small for worms but big enough for animals and other projectiles;
a worm hit by the beam will get nudged slightly (like the prod);
the laser beam can be reflected off terrain on low angles, but damage and burning through terrain will be reduced;
oil barrels and weapon crates will heat up and explode (like flames do).

2) Sheep-on-a-rope:
Bring it back!
Possible mechanics:
--oldschool, like in W:DC with the worm firing a rope and the suddenly appearing.
--new school (consistent with other sheep-type wepaons), where the worm drops a standard sheep and on the second press of the button the rope fires from the sheep, possibly straight up (the also supersheep launches straight up).

2) Black gun:
Use it as an "Gamma ray gun", as previously suggested by Run in his Worms Unlimited proposal.

A two shot weapon that can fire through terrain;
Damage maybe lower to the shotgun;
oil barrels and weapon crates should heat up due to the high energy gamma rays;
radiation sickness (poison effect).