I hate to point this out, but there's already a custom lego flamethrower available from a store called Tiny Tactical.
1. As has been said any time someone mentions that someone else makes an item that was suggested, just because one customizer makes a particular item does not mean that all others can not make a similar item. In fact, I am fairly certain there are quite a few customizers who make swords and military helmets. Spirit of competition and what not.
2.They didn't say a full flamethrower rig, just the canisters. We can supply our own hoses and such.
While I have no suggestions for new ideas, the idea of a jet pack or canisters is pretty sweet. Even I could think of uses and I focus most of my building on fantasy or feudal Japan.