Its always fun to think about new weapons, and everyone's noticed there's only four medium weapons currently... so why not brainstorm for fun?
Trident Medium Harpoon Gun. Single harpoon that has a much stronger lasso effect than its light version. Longer duration, and can reel enemies in with noticeable force. Very long reload.
Lighthouse Medium Flare Gun. Lens flare effect that can obscure the vision of close-to-projectile ships, and has the same effect, albeit much larger, as its light version in smoke. Larger AoE, same damage, longer reload.
Maelstrom Medium Gattling Gun. Wind up time that increases fire rate the longer the trigger is held down. Wide spread, double barrels, larger clip, approximately the same damage per bullet, perhaps less, as the light version.
Orion Medium Rocket Launcher. Quicker fire rate than the light version, slower projectile speed, no projectile drop, large explosive radius, more damage, same clip size, longer reload time.
Basilisk Fang Medium Flamethrower. Smaller clip size, shorter range, smaller AoE, increased fire rate and damage.
Any other ideas?