This produces a few problems:D20 SRD wrote:When making a melee attack, you get a +2 flanking bonus if your opponent is threatened by a character or creature friendly to you on the opponent’s opposite border or opposite corner.
1. "Friendly"? Are we going to get into a similar problem 4e Dark Sun had with Defilers, where allies were "green dots"? For instance, if I'm fighting an behir and the NPC rogue I hired back in town comes up behind me and tries to remove his employer's spine, should he get a flanking bonus? By RAW, he would not, but it would seem that flanking represents the defenders inability to keep tabs on both sides of him; hence why 360° Vision negates the modifier for flanking.
2. When should a character or creature count? Certainly that flea standing behind me is my enemy, but should it provide a flanking bonus for the Balor on the opposite side? If I'm fighting with my back to a table, which, unbeknownst to me, has an animate lampstand who hasn't moved yet, should the lampstand count for flanking?
My attempted resolution: Rejecting a Flank.
At any time you may choose to reject the flank of something, causing them to no longer count towards flanking, but if they do attack you, they are treated as Invisible (denying you your Dexterity bonus to AC) to you, since you were ignoring them.
By default, you are rejecting the flank of anything that is not obviously and enemy, including inanimate object and party memebers. You could choose to watch them more closely, but this has a cost: if you watch your back against that NPC rogue from before, the behir would receive a flanking bonus (but at least you wouldn't lose your Dex against the traitor).