As an alternative that provides flanking, but doesn't result in conga-line or trivial advantage:
Flanking
A creature who both starts and ends their turn surrounded becomes Flanked until the end of their next turn. While Flanked attacks on the creature have advantage.
A creature is surrounded if there are 2 foes of the same size or larger on opposite sides of them. For every size smaller the foes are, it requires 1 more creature to surround them.
...
This is designed to encourage mobility. It rewards threatening a flank with forcing the target to either suffer from advantage or do something about it.
Getting out of a flank can involve moving and suffering opportunity attacks, forced movement on the surrounding creatures, both of which make the battlefield more dynamic.
Flanking
A creature who both starts and ends their turn surrounded becomes Flanked until the end of their next turn. While Flanked attacks on the creature have advantage.
A creature is surrounded if there are 2 foes of the same size or larger on opposite sides of them. For every size smaller the foes are, it requires 1 more creature to surround them.
...
This is designed to encourage mobility. It rewards threatening a flank with forcing the target to either suffer from advantage or do something about it.
Getting out of a flank can involve moving and suffering opportunity attacks, forced movement on the surrounding creatures, both of which make the battlefield more dynamic.