The first priority will probably just be dealing with walled-off hallways and stuff like that though -- things that are really easy to do that kill off entire sieges. I guess it's entirely possible that they wouldn't be able to find you if you hid behind 10 meter thick walls hidden off in random places, but then it should be possible for them to live in your upper levels for years while you work away tradeless on mushrooms down below (although there are major obstacles to that that make it non-practical for the dev page). They'd probably just leave after looting all your exposed items and slaughtering anybody left outside (including any entity pop infrastructure you've got out there). Although once they can get through a single wall, they can just dig ambitious tunnels for you at random I suppose. As long as walling yourself in has reasonable results, I'll be happy with however they handle it. Right now it's too much of an exploit (of course, the whole "digging invaders" is enough of a touchy subject that it's explictly stated as optional on the dev page, and how you handle exploits is up to you at that point).
I would propose 2 systems to distinguish normal invaders from game ending HFS. It's realistic that if you hole up behind really really thick walls and never trade, then normal invaders don't have a way to realise there's dwarves hiding so they should leave you alone. At the same time, HFS shouldn't be stopped by walling it off no matter how thick the walls are.
Actually the HFS can have an alternative to digging to get around the current "wall off" exploit: what if they could pass through walls and floors? That is, they swim through rock just like they swim through magma? Then HFS would truly be the game enders and the existing infinite spawning of them would ensure forts die no matter what.
Regarding Night Creatures, assuming we're sticking with humanoids (as otherwise mythologies from around the world will have so many possible examples we'd be flooded - e.g. the one-eyed hopping umbrella) there's plenty room for a restricted random generator to work with: do they eat corpses, merely suck blood, steal souls, mutilate the deceased, infect others? How far have they rotted away and is it homogeneous or not (e.g. skeletal arms with zombie torso)? What do the locals call them? How aggressive and territorial are they? Do they have symbiotes? Side effects of being around them or getting some action done by them? DF is destined to be a fantasy world generator and Forgotten Beasts are the first sign of the RNG being used to invent new fantasy creatures. Somebody in our human history came up with the fantasy of vampires, zombies and such. DF will generate new fantasies in each world and we'll remember them if we interact with them in a memorable adventure (or fortress).
anyone else notice the last item on the list was boats and moving FORTRESSES!!! Bloody want my fortress to be able to stand up and walk away now...
I just know someone somewhere will design a fortress that can transform into a giant robot once some of these dev list items are implemented...