My group has been tossing around ideas for a variant rule for flanking/outnumbering for our next campaign. The idea we came up with is somewhat inspired by the game Blood Bowl:
When making a melee attack, each free ally adjacent to your target grants you a +1 bonus on your attack roll. A free ally is one who is adjacent to your target and not any other enemies. Incapacitated allies do not provide this bonus.
What do you think? This is for a 5e game, but I think could work for any edition of D&D/PF