I'd be careful at allowing "flanking" to grant advantage, for two reasons. First, the rogue's sneak attack is (currently) pretty potent once he hits level 2, roughly equaling the damage output of the fighter (making him feel even less special). Secondly, it therefore reasons that monsters should get advantage for flanking as well.Originally Posted by The J Pizzel
Given the wide range of hitpoints in the Caves of Chaos (monsters tend to either have less than 10 or greater than 50) the bonus sneak attack dice aren't going to do a heck of a lot of good against a kobold or goblin, but those same kobolds and goblins are going to be much more deadlier.
If you wanted to houserule something, do something like allow characters to make a Charisma or Dexterity check (DC 10) to feint in combat as a free action. Success grants them advantage, failure grants them disadvantage.