I agree with you that a lot of monsters do not have any unique abilities. I'd like to change this by 1) adding more monsters 2) and more importantly make sure that the ones already in the game feel more unique. Please have a look at (SPOILERS!!!!)
http://files.dwellergame.com/db/monsters.html and suggest ways to improve the existing monsters.
Some comments:
Mind (f)layer. I guess you missed an 'f' there? It would probably be quite easy to add a monster that turns friendly, neutral and charmed monsters into hostile ones.
Sandworm. I like the idea of adding monsters that can exist only on certain types of terrain. It would be a bit difficult to code though. I have full support for monsters that only exist on certain levels (sewers, labyrinth etc) so once I add a Water themed dungeon branch I could add Mermen, Crocs and what now, and the same goes if I add a sand/dirt/earth based level.
Hydra. So far Dweller doesn't have any large mythical beasts (sans the Minotaur) such as manticores, hydras and dragons and I'm not sure about adding them... I could easily add a cleaving/smashing multi target attack to the Minotaur, Troll or Rat King.