Who remembers Aloshi's homing missile from v8? Make the piranha like that except it dies when you dont kill people for a while.
Yeah, a slight homing effect would be nice for this weapon. If the piranha misses a player and hits a wall or something it should explode damaging anything around it.
Another idea for people to consider.
Mouse1 - homing piranha
Mouse2 - non-homing piranha with a bigger explosion on a non-player surface