I'll cut straight to the point on this one. There are certain creatures that should, eventually, be randomized more, and certain ones that should be randomized a little less.
Creatures that should be randomized more include different styles/flavors of were beasts and vampires (both of which Toady mentioned in a previous dwarf fortress talk), but also things like dragons (western vs. oriental).
Creatures that (I think) should be randomized less: a little less variation in the forgotten beast tables (and by little I mean little) might make it a bit more challenging, so that a forgotten beast made entirely of snow doesn't occur. Call me hardcore, but I think that's not enough fun, in the dwarven sense (though fire forgotten beasts are very !!FUN!!). The HFS creatures also, in my opinion, should have fewer options, because it gets annoying to have to get used to a whole new pack of creatures every time I dig too deep. The third: animal people. I know they a aren't randomized, but they occur a lot in savage biomes and crowd out all of the creatures that would naturally be there, like giant wolverines. Perhaps during world gen a certain number of surface animal people types would be selected rather than all of the types.
If anyone has any other creatures they want to add to the list feel free.