So I think I've found my dream site. I did a 3x2, but you could get everything in a 2x2 if you wanted.
It lies at the border of three biomes:
The Pregnant Hill, a joyous wilds temperate shrubland;
The Water of Stenches, a terrifying temperate ocean that never freezes;
The Malign Jungles, a terrifying temperate conifer forest (covered with evil vegetation).
Surface is 3 z-levels, relatively flat (I would prefer it to have a little more surface variation, but that's hard to get in such a small embark). Whole thing is covered by an aquifer, but it's only 1 z-level thick in some places, and there's a bit of rock access before you get through it.
Both the ocean and the forest have evil weather-- and they have different evil weather from each other, although I haven't yet figured out what either one does. Both are also reanimating biomes, so for instance, the earliest wildlife I saw was an undead hammerhead shark scooting around on the ground. Also plenty of undead fliers, undead snails, probably a tremendous variety of wildlife.
I've got sand of multiple varieties, fire clay, shells to be had in the ocean, flux in the form of dolomite, and iron in the form of hematite. Trees for the taking on the surface, if you aren't afraid of the walking dead. Of course, there's magma, if you can reach it, and water for the taking, if you can desalinate it. Access to humans, elves, dwarves, goblins, and kobolds (man, undead kobolds are not fun). Elves so far have brought an emu, which I declined. Not sure what else they might bring, or if unicorns are in the works. Playing with a 400 year history and an abridged history seed so I could regen to a different year, but a quick look through legends doesn't show me any interesting wars fought by the elves or the humans so far. Not sure about wind, but who needs wind when you've got an aquifer? Haven't fully explored the third cavern, so dunno if there's a curious underground structure.
For me, this is pretty much the perfect mix of difficulty and completeness. Only thing missing (maybe) is curious underground structure, wars, and nearby towers. If you want extreme challenge mode, you could always cut out the joyous wilds and go for a fully reanimating embark.
Is this sort of thing anybody else's cup of tea?
EDIT: Apparently, one of the forms of evil weather causes drowsiness, loss of vision, and fatal full-body necrosis. Yum!
EDIT2: Not necessarily fatal, although it leaves the victims blinded for the rest of their miserable lives. The second kind of deadly weather makes unholy gloom husks. Oh boy. Thankfully, that weather doesn't occur often... The humans come riding horses and camels, the elves, war rhinoceroses and war polar bears. Still haven't seen any unicorns. The goblins are fond of giant olms and beakdogs, and bring trolls.