Fire just makes the most sense, since it's a light based weapon.
Personally I don't see it needing a 'buildup' time, since it's just reflecting and focusing light. If it was fire damage, the stacks would naturally build up and deal more damage over time, just like the flamethrower. Essentially it's heating with focused light instead of combustion.
That's not to say it couldn't have a buildup, but I don't think we have a mechanic for that in the game currently. Right now we could easily use the arming time and shell life to limit it's effective range from being too far or too close.
Of course the damage types, the effective range, the arcs, and the size of the gun (although I think we're more or less in agreement that heavy would be best?), clip size, fire rate, reload, and projectile speed are all easily debatable.
I think we all have a vision of what kind of gun we'd like to see, but we also have to consider where it would fit in the array of weapons. Would it be redundant? Too difficult to use? Would it be overpowered when combined with another weapon? (ie: if it's piercing, what about combining it with a flak or hwacha?) Would it replace other weapons that area already in the game?