This would be a big, slow moving ball of ionised gas, but it would have 1 trick, not only would it do tremendus damage to the target, it would also do, lets say, 10 damage to all other ships that are short range of it. So it would be sort of like that BFG from quake 2 or Doom 3.
This would be best if the ship that has this weapon would cost credits

or even a natural things like a bhall of gas that is a star in the making, its so hot it does huge damage but is easy enough to move from if you have the intelligence.

Well think about it this way, sort of like a metal ball with huge voltage in it and if u put your hand to close a spark jumps from the ball to your hand burning it a little
I thought sort of in that way, so gas perhaps wasnt the right expression maybe a electrically charged dilitium ball would be more like it lol

ooh i can see a barrel like gungunship. ( a floating piece of pipe like ship maybe) with warp nacells on the side that creates a  plasma ball in space and hurls it at great distances like a comet.

i don't think damaging everything near it can be used due to teh fleetosp engine.
 
unless one could treat it like a neblula