I think a great addition to Skyrim would be the ability to turn roadsigns so they point to the wrong city. And a few minutes later an NPC will come along, look at the sign, scratch his head and say "Wait a minute, thats not right... or is it?"

Does anyone else think this would be fun?

Playing pranks on NPC's? lol Well there would be a lot of humour in that, but I don't think it'll make it in... sounds like a lot of programming involved!

Sorry, I can live without that <_<

I wouldn't want to play a prank on some poor unsuspecting NPC that I can't even talk to.
How else am I going to know who deserves to wake up with ghosts in his room?

Pranks have always been fun, however you make them. Of course they never react like you'd like...

I would rather steal everything inside their house and put it in a barrel

That sounds like a scripted possability, and the sign being the activator (not really that hard to do) to make it dynamic and have more than just 2 outcomes depending on the NPC would require lots of programming though, but these types of interactions are done everyday in videogames