I once saw an actual fight at a park. 6 guys were trying to beat up one guy who happened to be a martial artist. They never got flank on him. He hit them. They hit him. But he constantly moved, never staying in a single place for an instant. Kicking, punching, running, retreating. I never saw the outcome of the fight (he was on the other side of a waterway and went out of view), but I do know that for a real melee fighter in the real world who has room to maneuver, flank is very difficult to get.
D&D does not have this "constant move" capability of real life, so RPG flank shouldn't mean much of anything from a plausibility POV. It's not like the NPC is just going to stand there and have one guy in front of him and one guy behind him.
Actually, that's why I've gotten away from the mini game on the table. I'll use minis as a visual aid, but not a combat resolution system. Using minis essentially eliminates the constant motion of combat, where TotM lets you describe a combat in real terms, with lots of motion.
Just because everybody was moving around doesn't mean that they were never able to flank him. The game uses the term flank, and specified that you had to be on opposite sides of the target, but the reality is that multiple trained opponents targeting the same opponent can work together and set up opportunities that give them an advantage. And I think it would be very difficult (at least by observing one fight) to determine if any of them found it easier to hit him because there were more of them. To hit one person requires him to turn his focus, however quickly, to that opponent. In that moment an opponent on the other side has an opportunity that would not be there if it was a one-on-one fight. That's why two or more against one is usually viewed as an unfair fight.
D&D isn't the best combat simulation to account for all of the variables. So why try to include one of those variables? Well, we include cover, because it's easy to determine and has a measurable effect. I think that flanking is another tactic that fits that description. And since it's already described for rogues, and any creature with pack tactics, it also has precedence in this version of the rules.
So far, my players want a flanking tactic/rule. So that's why I'm trying to accommodate them. The help rule works fine, but we felt it was a bit restrictive in that the helping character couldn't attack at all that round. Which also didn't make a lot of sense, because if you weren't really a threat that round, then they could focus on the other PC. I also don't want to take away the benefit that some creatures have, which is why I added the restriction that you couldn't be near any other hostile creatures.
I haven't really run into any real issue with the rule as we've been playing it. But I also enjoy these types of discussions, and since a number of people think it's too powerful it makes me think some more...
So here are the questions that I guess I'd like to answer (at least for myself):
If everything else is evenly matched, does an attacker have an advantage when they outnumber their opponent in direct melee combat?
If so, what's the advantage?
Is it easier to hit them?
Is it easier to cause more damage?
Does it give you more opportunities to attack?
Do all of the attackers gain the benefits, or just one?
Do you need special training to gain the benefits? Remembering that all PC's are trained in combat, that is they have proficiency, but do you need a special ability like a feat?
Here's an interesting article:
http://www.wikihow.com/Fight-off-Multiple-Opponents
One thing that I find particularly interesting (and may be what you actually saw in the fight) is to keep backing away to prevent them from surrounding you. So one option would be to use the same mechanism as Pack Tactics and/or Martial Advantage (which is basically sneak attack). I could go with a feat that would allow its use, in which case only the character with the feat gains the benefit. I might also add an option to the Mobile feat to avoid being flanked, or perhaps grant a Dex save.
That would be the second option, to allow anybody to use the tactic, but that it requires an opposed Dexterity check to gain the benefit.
I'm at a stage where if the benefit isn't really worthwhile, then there's not much point in creating the rule. Both Martial Advantage and Pack Tactics make it worthwhile. If the tactic can't provide a similar benefit, then I'll probably just scrap it.
Ilbranteloth