I'd much rather have the possibility of having no demon at all -- and by extension no adamantium -- and have the river, chasm, and magma stream all appear at variable depths and in variable order. To fix the obvious farming problem, perhaps there could be a couple springs of random direction and length inside the mountain that lead out to the exterior river.
Maybe what I'm getting at here is that I'd like fortress maps to be a lot more random in general, moreso than I'm complaining about the hard-coded guaranteed end to any fortress...