I've been pondering some designs for several mega weapon projects, that is, mega projects designed specifically for fighting surface enemies.
There is an elegant device called an Dwarf orbital strike. I'm not sure how extensive these are, most I've looked at seem to deliver magma to a single spot right in front of the gate. I've been planning for a device that could deliver magma from an elevated reservoir to the center of any 5x5 square on the surface from 20zlvls up. Basically a huge areal grid with notations on the ground of the X,Y coordinates. It would be very precise, capable of dropping a stream of magma on or next to a single goblin anywhere. It could also, amusingly, be used to make a ring around a squad of enemies. If you were adventurous enough with a fast computer you could use it to support friendly units in the field.
For a valley fortress I've thought of a multi-shot FTW device. Basically you hollow out the walls of the valley on each level and use 2x10 raising drawbridges where people would normally use floodgates. Instead of having magma stream out of two or three points you essentially have the whole end of a reservoir stretching the entire length of the map suddenly open up. This would result theoretically in a gullywasher of magma. How many shots you get is dependent on how many levels you have available to hollow out on each side of the valley. Why you would need a multi-shot insta-kill region-sweeper? Why not? Bragging rights? I also imagine it would be handy with the Fortress Defense Mod where you can get chains of sieges.
I've thought of a massively impractical mega weapon called
Arclight . It would consist of row upon row of collapsible ceiling set up in the middle of a field. When dropped in sequence I imagine it would resemble a B-52 carpet bombing. A one shot weapon with a hideously long reset time depending on the scale (I imagine 20 rows 20 squares long). Likely to crash my computer. It would be funny to employ it in a community fortress though.

Or I could just be a boring wiseass and cover the entire surface with 10/10 weapon traps.