Barrow-wights are quite strong and scary in the mod already, but I feel they are still weak considering all their scaryness. Also, I don't think barrow-downs should be an easy source of of valuables, but rather a place most people want to avoid.
Cold be hand...Edit
Barrow-wights should get a new ability to give slowness and mining fatigue (freeze) to enemies. I'm pretty sure this is canon because frodo was freezed inside the barrow as I remember. They'd also make a noise when they freeze you similiar to the current whispering ambient, but the voice would be more similiar to the description in the books.
I had an idea that barrow-wights should be enable to pass trough walls, like the ender dragon (except not break any blocks of course) to stop farming them in the way of poking them to death from a closed space. The reason would also be to make gaining mithril more difficult. Not sure how canon this is however.
Barrow-wights would remain at 50 health and their current attack, but they will get 11 defence points. The wightbane modifier would enable a weapon to completely ignore these armour points.
The current black screen and fog things are scary, but I don't find the black screen thing quite that good yet. I'd say it would be better if your FOV and the blackness would constantly change during it, fading (black thing)/going normal for a moment (FOV), then agian taking up almost the entire screen (black thing)/going extreme (FOV). Barrow-wights could also get animated textures where their entire skin goes black for a moment, then starts turning greener agian.