Some sort of flanking rules seem inevitable. I realize there was no flanking rule in AD&D (or was there? I don't remember.) but 3rd Edition opened the box and I can't see putting the genie back into the bottle now.
The question seems to be the implementation.
Our group can see it being done in one of two ways.
#1: The obvious way. The attackers get advantage. This makes the surrounded individual (the poor sap) unlikely to stay standing for long.
#2: The interesting way. The surrounded sap suffers disadvantage on his/her/its actions. This seems counter intuitive. I mean, the attackers have set themselves up around the poor sap; therefore, they should get the advantage for their efforts.
But, look at it from the poor sap's point of view. He/she/it is surrounded by enemies with sharp knives/teeth/claws. Naturally, one is going to be a bit timid in that situation; attacks will be halfhearted to avoid overreaching and opening oneself up for attack. Hence the sap's disadvantage.
I can see WotC going either way. I can also see them coming up with something else entirely. The only thing I can't see is there not being any advantage/disadvantage/other for being flanked or surrounded.