Flanking is an optional rule in 5e, and we’re going to implement a modified version it for our game.
Flanking will only be possible when two characters are fighting a single opponent with no other opponents within 10 feet. My reasoning for this rule is that in order to properly flank, characters need to be utterly free to concentrate and coordinate with each other, and on a complex, chaotic battlefield, I rule that this is impossible.
Flanking will provide a +2 To Hit bonus rather than the RAW benefit of granting Advantage to the characters. My reasoning is that because of Bounded Accuracy of 5e’s game design, +2 is a significant bonus and, at the same time, does not negate the Advantage mechanic that one or both of the flanking characters may generate due to abilities or circumstances on their own like it would if implemented as per the rules as they’re written.
I like the thought that a Battle Master Fighter and her Barbarian boyfriend might flank an Ogre for some fun and flirtatious mayhem… the Battle Master successfully Trips the Ogre (getting a +2 To Hit on the attack from flanking), then follows up with another attack (or three if the Fighter has multi-attack and uses Action Surge) that has Advantage from the Prone Opponent rule in addition to the +2 for Flanking. Then the Ogre stands and attacks the Fighter, followed by the Barbarian attacking Recklessly (thus with Advantage), also with a +2 To Hit.
This rule also makes the Rogue much more deadly when backstabbing… as it should be.
Wolves (and some dog breeds – most notably the Rhodesian Ridgeback – an African dog that was bred to hunt lions) often take down dangerous prey (like frickin’ lions!) by surrounding said prey and attacking from behind… as the prey spins to defend against the attack, it just opens itself to more attacks from whichever wolf is now at its rear.
What this means for our game is that once two characters have set up a flanking attack against a given foe, any other characters that join in to the attack will also be granted a flanking bonus regardless of whether they’re technically in flanking position with anyone else. The notion is that once you’ve got someone who “doesn’t know where the next attack is coming from,” adding more folks to the list of potential attackers just makes it worse.