Something that should count for a heck of a lot in a fight is numbers, but this is something that's basically irrelevant in D&D right through 3E. 3E added a "flanking bonus" of +2. I don't think that's nearly enough. Fighting while outnumbered should be terrifying.
What I propose is this. The opponent you're attacking has to beat your Defense to hit you. Anyone else fighting you only has to beat the minimum Defense score (9 in descending style systems, or 10 in ascending).
That means that if your Defense is 3 against one opponent (descending Armour Class style), it's 9 against a second or third. I think this is probably still a little generous, but it has the benefit of simplicity. This would make the party really think before engaging a superior force
(as they should), and seek every opportunity to isolate enemies and
defeat them in detail (as they should).
I also see there being a level bonus (maybe every odd level for Fighters) that lets you fight an additional opponent without penalty. Or maybe a feat. The point is, being able to fight multiple opponents without penalty should be a hard-won privilege, not the right of every stable hand that picks up a sword.
This elegant treatment is made possible by divorcing defensive abilities from armour.