Whenever a single defender faces more than one opponent, the attackers add +1 to the attack and damage dice for every foeman beyond the first. For example, a 1st level fighter faces off against 3 orcs.There are two foemen (orcs in this case) beyond the first in this case, so each of the attackers gains a +2 to the attack and damage dice.
However, this rule is offset by the fact that a fighter is able to defend against a number of opponents equal to his level, without penalty. Taking the previous example, we'll make the fighter 2nd level. Facing off against 3 orcs means that there is one foeman beyond the initial two against which the fighter is able to effectively defend. Thus, each orc would be granted +1 to the attack and damage dice. If the fighter in question was 3rd level, his 3 attackers would have no advantage on either the attack or damage dice.
The rules for flank and rear attacks are relegated to special circumstances.
NB. The maximum number of foes able to attack any given single defender must be kept in mind.
With the above rule, lower level attackers gain an advantage when fighting against a single defender by "ganging up" against the defender, with the advantage growing exponentially as the number of attackers against a single defender grows beyond the ability of the defender to handle multiple foes (as determined by level). This is offset by the level of the fighter defending against multiple foes. So, Conan at 1st level would be at a great disadvantage against multiple foes, while Conan at 9th level would be able to defend against numerous foemen with no disadvantage.
One thing I haven't decided upon is whether the number of foes against which a fighter can defend against should be capped at a certain point. Right now, I'm think that name level (9th) would work well as a cap. Also, I haven't decided what to do with clerics, and possibly thieves, if anything. I have considered allowing clerics to defend against an additional foeman beyond the first at the rate of 1 every 2nd level (i.e., 2nd, 4th, etc.) and thieves at the rate of 1 every 3rd level (3rd, 6th, etc.), but haven't decided on this yet.
Overall, this rule developed as a result of thinking about the one minute combat round. The rules for flank and rear attacks didn't make a lot of sense to me given the one minute melee round, wherein it is assumed that attackers and defenders alike are making multiple feints, moving around for advantage, etc. during that long (in combat) 60 second time period. Unless every round of combat was broken down into discrete actions during segments, how is the facing of a defender determined at any given time against multiple foes? How does one keep the notion of flank and/or rear attacks as part of the 1 minute combat round when any given battle is going to be quite fluid during those 60 seconds, while maintaining the abstraction that is inherent in the system? There are some folks that go the opposite direction than I have gone here, reducing the length of a combat round significantly (usually to 6 or 10 seconds), adopting a defacto segment by segment approach to combat. Desiring to keep the abstraction, I've gone the opposite way with this rule. Facing and position during the 60 second combat round is completely abstracted, with the flank and rear attack rules used as special circumstances, or exceptions to the rules, as I believe many of the rules for combat in First Edition are intended to be.
Also, the notion from Chainmail where a fighter of any given level was considered to be equal to a number of normal men came to mind. Thus, a super hero (4th level) that is "equal to 4 men," gave me the idea that he could defend against 4 opponents without those opponents gaining an advantage against him. The "angry villager" rule came to mind as well, as did the First Edition rule where fighters are able to make a number of attacks against 0-level men and monsters equal to his experience levels.
At this point, however, we haven't used this rule in game play, so if anyone tries this out it'd be great to get some feedback on it. We're going to give this a go this weekend or the next, so I'll have some data after that.