Keep it simple: I mean, that's why you chose the shotgun, right? Find the bead, pull the trigger, target goes down...extra sights and other accessories basically make the shotgun into a 4 to 8 shot manually operated rifle, while slowing you down.
That said, a tactical light is very useful. If you illuminate someone while loudly racking one into the chamber, and yelling "STOP OR I'LL SHOOT", it should resolve the situation one way or the other.
Good lights are expensive, though. It should be on the fore-end (don't mess with barrel-mounted ones, because you want to be able to turn it on and off without any cords dangling, etc.).
I'm not a fan of extended magazines because they make the gun more awkward, and can be a source of malfunctions.