Just a random thought... I wonder how people would like it if getting near SS2's psi reavers had a disorenting audio/visual effect similar to HL2's Combine Advisors, or STALKER's Brain Scorcher. Might make them a bit more properly terrifying to fight.

I figure psi reavers turned out more lame than Irrational planned them to... basically awkward slow-moving turrets that mumble every now and then. Compare to the experience of encountering a hammer haunt in Thief.

Sounds "nice", some sort of tunnel vision would definitely appear apropriate.
You could even make the player just a little bit slower, since they have to concentrate on keeping the Reaver's prodding PSI-Whatevers out of their head...

Frictional used this mechanic to great effect in all their games and it worked wonders but it's tricky to pull off seamlessly.  What would the engine allow in terms of audio/visual effects?  Another idea is to do this sort of thing only for the greater reaver, which I always felt was terribly nerfed in the game and which should have been a more developed enemy to show the end of the annelid lifecycle.  Sure the brain and a projection is all that's left but that's almost intuitive, if not plausible.  I wanted to see what the next stage would be after that!

The psychic attack effects are pretty much just sounds and player view overlays. Dark can handle that stuff easily.

IIRC fighting the disappearing projections felt like fighting multiple reavers, since the game has this tendency to appear new enemies around the corner that all look the same anyway.
Would be nice if their dis/reappearance happened more often but also only in close vicinity. Zap-zap!