Well, I have had this idea for a VERY long time.
I bought a dwarf flame cannon and have always planned to convert it into an earthsaker with a bit of changing to the cannisters on it (to make them more like gas propulsion for the shell or soemthing, not sure what) and put a marauder drum or something in the mouth as the ammo. This would be an Earthshaker.
Another option I made for myself was to remove the tanks and stick some kind of rocket head in and use as a death rocket.
A third idea I have had is, probably using the new dwarf bolt thrower, replace the bolt with a rocket and use as a death rocket. Could always be used as a bolt thrower still.
The most common bolt thrower conversion that I know of, and one I thought of before I found out about it, is to buy a goblin spear chukka and remove the “teeth” part. Convert the goblins a little if you want and hey presto!
Any other ideas on warmachines?