While we're suggesting food for thought, see Modos RPG:
What I mean by flexible defense
Most rpgs that I know of handle defending against an attack like this: Attacker needs to beat a target defensive value in order to hit. 90% of the time that number doesn't change.
Attacker needs to beat the defender's roll + defender's bonus. That's a different number every time, unless the defender A) doesn't defend or B) takes half on the roll.
Sure, sometimes it's different based on factors (3e famously has a few ACs depending on what was going on), but it was still largely the same number whether you were being attacked by one person in a round or ten. Sometimes you could boost that target # via several means (spells, dodging, etc.), but again, the number stayed pretty static regardless of who or what was actually attacking you.
Defenders, like most everyone else, get three actions. So the first three attackers (attacks) would attack the same defense roll, and the rest would get free hits. The defender could boost the defense roll (target #) by having some sort of advantage, either from the situation or by using an action to create one.
If you have a pool of defensive ability in a round, maybe you don't worry so much about the mooks attacking you, but focus your defense on a tougher opponent.
Disregarding mooks is a good way to get trounced. BUT, if that tougher opponent is the leader or morale support, a quick takedown might scare the mooks away or into submission. Also, the tougher opponent might be capable of doing greater-than-one damage per attack, while mooks are probably doing the bare minimum, so it's a better idea to use actions against those tougher attacks (if the plan is to occupy the tougher opponent).
Why I want to do this
It's just one way of many to allow martial and mundane PCs have more stuff to do in combat. One round they may act like a tank, while the next go on offense. It models the verisimilitude I prefer in how a combat encounter looks like against multiple opponents--shields can't block every attack at the same effectiveness and being swarmed matters (this part is subjective, I know). I think it adds a new facet to the combat encounter that's largely overlooked.
More stuff...in D&D, I'll assume. Other games seem to be providing plenty of options, here. Modos RPG provides yet another option - defensive posture - which allows a defender to get behind allies, behind cover, or further away from the fight, which gives attackers another choice: do I fight the nearby enemies or spend time/energy getting a better position on the harder-to-reach foes?