The howitzer would be hilarious to have on a ship. Pretending it would fit and such. I imagine that firing it would propel your ship backwards pretty violently.
I know that the original Guns of Icarus had Tesla weapons, but I'm fairly certain they decided to go a different route in GoIO and not put Tesla weapons in. It used to be a damage type, but was replaced at some point (with shatter I believe). The harpoon was an electric harpoon at some point, but has been changed; so it's pretty safe to say that they probably won't make an appearance in Skirmish. Adventure mode may be an entirely different story though.
I love that image of the GodLamp, and think it would make an epic heavy weapon. I could see it firing off a beam of light that does massive fire damage. The ammo might have to be the gun 'overheating' though or something like that. It might have to work through some kind of powered device. I don't think reflecting sunlight would work very well, what with all the foggy/dark maps and the balloon from the ship getting in the way.
As for beams of light themselves, they don't operate like lasers (unless you want to make it a laser cannon?). One example is the magnifying glass and the ants. Any sadistic child who's done this (all of you, don't lie) knows you need to hold it a certain distance from the ant, at the right angle. This is to get the ant at the focal point, where the area of the beam is the smallest, maximizing the energy/area. Then you get a cooked ant. Too far away and the light scatters about, too close and the focal plane is too large. You'll warm the ant up a bit, but you're not going to cook him.
Which would make this weapon very interesting. We already have weapons whose damage is based on relative distance. This one could be an extreme example of that. At the furthest point of it's effective range they could be taking max fire damage, while moving closer reduces it up until the point there is none. The gun could use some kind of light or power source as ammo instead of the sun.