Maybe something like this?
As you can see, (kinda) the Flamethrower is shooting a liquid arc.
The arc would be effected by gravity (maybe like the mortars) so that could limit the range, but still give it some massive distance in the right hands.
The stream could travel pretty slowly, so it wouldn't be OP.
I dunno, I just kinda like the idea of a gunner screaming "OH GOD. HEAVY FLAMER. GO UP GO UP UP UP UP UP UP." As a stream of death approaches him.
But, as a Cake, I support a heavy flamer in any shape or form.