Artillery cannon. Arming time and distance selectable by gunner. Shell explodes at that distance with a large AOE blast to say the size of flame thrower range. So you don't have to hit a vessel directly, just be near it. Could even give it a proximity range. Shell arc is greater than LJ. You cannot aim it directly at a vessel. Weapon would be intended for usage with a rangefinder. Advanced weapon for sure. Would take 2 to operate and a gunner to plan where the shell will fall in advance to catch a vessel.
Actually... this.
I'd like some sort of artillery weapon that explodes into smaller explosive/fire shells when it reaches its arming range. Very high arc, as you suggested, with the spawned projectiles randomly shooting out in a small-medium sized aoe around the shell. In this way it can be used at long-range in innovative ways, for example, firing over walls in Paritan or Canyon Ambush to scope out enemy locations or force them to move.
It can also do minor impact damage if it doesn't arm, giving it some utility on a spire, for example, which might be trying to knock a charging metamidion out of arc. I can't see that being very OP, but I can definitely see some builds making use of it. Would definitely make some ships like Spire or Galleon more viable on CK or CP maps, by being able to support at long range by bombarding teams camping points.
TLDR - artillery weapon with fragmentation effect when it reaches arming range, minor impact damage otherwise.