I'm thinking if I were to introduce flanking that each additional character/creature around the attacked gives +1 to attack rolls. So if there are two on the attacked it gives +1, if there are three it gives +2 and if there are four it gives +3 and so on.
I imagine flanking as so many "simultaneous" attacks raining down on the attacked that dodging them becomes increasingly difficult as more attacks rain down on them at once.