I feel non need for a light to upset what semblance of balance there might be - but that is because it is only my wife and myself, no kids, dogs or other and there is plenty of ambient light coming through the windows from the neighbors' curbside street lights....That said, IF i were to add one and only one thing extra to a standard stock 870 or 500, a light comes in as a tie for first with a side saddle.
Night sights?? what for - if you can easily ID the target, then there is enough light to fire, if you can't see the target, night sights aren't going to help anyway - your focus should be on your target, not your sights - this isn't long-range gunning. Most shots INSIDE a house will max out at 30 feet or less - 10 yards. Mag extensions sounds nice, until you try to move the gun - then that extra weight can become a hindrance and not a help - especially if you have not trained and practiced with that. A lot of folks seem to think they will need a bazillion rounds to ward off an intruder - stats show in most cases NO shots were ever fired, and in the majority of those where a gun was fired, one was the magic number.
In an emergency scenario - no matter the type, KISS is your friend