I would like to see magic trapping doors again too. In fact, I bet it would be super hilarious to make the "trapper" flag grey when the "trap-ee" opens the door in town. Who doesn't like seeing someone getting randomly guardwhacked?
Oh interesting consideration there. If trapped doors did 1dmg, the trapper would potentially flag. This could make for some interesting dungeon dynamics or public house shenanigans...
I don't remember how the flagging worked with it. I don't seem to remember getting a message that I was attacking anyone when someone busted through a door that was magic trapped, just like I don't get a message from a thief that snoops a bag. I think it had to do with the damage being so minuscule or non-existent. I'd have to look into the flagging aspect.
