Wow! That was a lot more posts than I expected.
Replying to just the ones on the 1st page:
Is this in addition to, or instead of Flanking giving advantage to attackers?
It is instead of flanking giving advantage to attackers.
I'd suggest changing the size restriction. Being flanked by halflings or goblins is still a threat, and still makes defending yourself/fighting back harder.
Yes, that's true, I included that caveat for this reason:
As with most flanking rules, this will make solo monsters easier to take down. A creature that can't hit isn't much of a threat.
For dragons, giants, and other big solo monsters, flanking would not be an issue. And it makes conceptual sense--dragons and giants have a longer than human reach and don't need to defend themselves from being stabbed in the same that, say, a duelist would.
I just want to make sure I understand. So if Eric is fighting two orcs (both within 5 feet of him) he has disadvantage on the orcs (because they are not flanked), but if Eric is joined by Sara, now they don't have disadvantage because everyone is flanked? Is that correct?
Yes, if Sara moves into a position within 5 feet of both orcs, that's correct.
If she moves within 5 feet of only one orc, only that orc will be flanked--which means that Eric would be able to attack the flanked orc without disadvantage, but would still have disadvantage against the other orc.
Similarly, the flanked orc would have disadvantage when attacking Sara, but not when attacking Eric.
I realize that's a bit cludgey and doesn't model real fighting particularly well, but I wanted to add a countermeasure for the flanking penalty without making the rule too complicated. I'm open to suggestions.
My suggested would be to add - When flanked, the creature also has 3/4 cover. This would add decision point to the attackers if they have any ranged friends trying to take advantage.
I like this suggestion. It's not to complicated an extension and it adds depth.
So, new rule:
"If a creature is within 5 feet of two hostile creatures of its size or larger, it is flanked. A flanked creature has disadvantage when attacking a target that is not also flanked and has 3/4 cover against effects originating 30 feet away or farther."