As Snail said, it's really more about the tactical niche it fills. You need to provide the player with interesting ways to interact with other fighters and capital ships.
Since you asked so nicely...
Sounds like the Laser Blossom from Allegiance: basically a deployable sentry gun that shot a ton of lasers in every direction. Great against capships!
Interesting because:
Most of the lasers/beams won't hit anything unless you deploy them pretty much right up against the hull of a large target. Basically becomes an interesting alternative to bombs. It would expire on its own after 4-5 seconds.
It would also be interesting to see Allegiance "combat pods" as well. Basically, deployable sentry guns.
Interesting because:
Combat pods have a time limit and expire (20-30 seconds or so). So, for example, you could lay down a net of these in front of a bomb run and catch them in the crossfire. Alternately, let's say you are in a dogfight with a lighter fighter that always manages to stay behind you. Drop a couple of these and that fighter has more to think about. Combat pods would also have light armor and no shields, so they are fairly easy to shoot down. They would probably be best with a high-velocity, low-damage, short-range laser.
Finally, I've always wanted the Impact Mortar from Descent 3 in Freespace.
Interesting because:
This would probably be used similar to the Infyrno: use it against bomber/fighter clusters or as a way to bombard capships. For those not familiar with Descent 3, this is basically a momentum-launched grenade (most of its velocity is inherited from the firing ship, creates a fairly large explosion either on contact or after 2-3 seconds).
