I’d love more audio hallucinations. “You hear glass breaking to the south” makes me check every time, just in case. Terrifying roar in a forest? Time to craft a new pair of pants.
I’d also love for Mi-Go, and hallucinations in general, to properly emulate NPC speech sometimes “You and I need to talk, friend!”, as well as for NPCs in need to actually scream and cry for help “You hear a young child crying.” Check it out, potentially save a small child for morale and a small material reward, or get your face eaten by a Mi-Go, or… Nothing was there at all. Except the hulk.
For visual hallucinations, can they be weighted based on current/nearby map tiles? Towns gives a bunch of more-standard zombies, giant wasps, forest gives Triffids, various wildlife, ants, spiders, fungi, Jabberwocks and so forth. Secondly, could hallucinations just “reskin” regular enemies? That child zombie you just melee’d? Actually a spitter, enjoy! Spitter you shot? Actually an NPC! Whoopsie!
Secondly would be mind screws. The Flesh Angel attacks! What, you’re taking damage? Omg! It’s real! Attack! The flesh angel disappears. Your health is restored to whatever it was that you lost, leaving only lingering pain behind. You mutate! You grow a spiked tail! Oh, it doesn’t do anything and disappeared with your hallucinations?
Finally, could “your mind make it real” sometimes? When the barriers between reality are so thin, your hallucinations could very well be supplying an actual conduit through which abominations enter the dimension, at least temporarily. These hallucinations might still be constrained (can’t bash obstacles, for example, so “hide in the nearest closet” until your mind stops being a dick becomes a potential strategy).
It needn’t be every time, that one in a million event where you were chased through the woods by a murderous multi-cooker for two hours is plenty enough to swear you off shrooms forever.