Ok we have buzz saws but i think dart traps. arrow turrets. and more should be in the game. put trap ideas in the comments.

Fire? I suppose you could allow it to burn away nearby blocks from the affected area.
Electricity? Shock the enemies making them vulnerable.
Oil/ Sludge? Slow the enemies making them vulnerable.

Water you short circuit, I guess. Not fond of this idea.

I suppose it would add to the tech tree, but you could add to the skills "Environment Resistance" as of course you'd be affected as well.