Things I want and tried to make (unsuccessfully):
Final Fantasy style bombs. (crashed the game to desktop whenever they blew up. This was before syndromes though, so it may be worth another try)
A flying wagon. (not sure if the hauling creatures need to be fliers as well to work, doubtful though since wagon pathfinding does not include sky)
Stirge (It actually kind of worked, but they were too small to damage through even light clothing, had to get a lucky hit on an uncovered bit. Proved to be free combat xp in adventure mode and not much else)
Any kind of heavily armored creature with directional weak points. (Closest I have is a lizard with thicker scales on it's torso and head, but I want direction of attack to matter)
A flying fortress race. (so many pathfinding issues)
Stuff that I want but know won't work:
Purple Worm. (No digging, and it's most dangerous part is the swallow whole ability which can't be done)
Final Fantasy Leviathan. (Not the same without being able to create a tidal wave, as a sea creature it would never be seen in fortress mode)
DnD Blue dragons. (no satisfying lightning breath)
Racial pets. (making stuff pet exotic makes it so not everyone tames them by default, but while the game is apparently supposed to have civs get a chance to tame local exotics and be able to embark with them, I haven't seen it happen once)
Uktena. Cherokee monster, a massive serpent with antlers and poison breath. If you look at it when it's sleeping, everyone you love dies. I'd also kill to see behemoth, leviathan, and ziz megabeasts.
I have done a FF behemoth at one time, but I haven't updated it to new versions. There was also some oddity with whether the front legs were hands or hooves. Not sure mine would have been the best example since I often have a 'good enough' mentality, but it is definitely doable.
A griffin
Done, outdated, but all it needs is some gaits, the speed tags removed, and it's name re-spelled.