Originally Posted by
Man_Over_Game
One thing I've used is +1 to hit for each other creature adjacent to that target that is its enemy. Deduct this bonus by -2 if the target has a shield, -1 if the target is wielding a different weapon in each hand, +1 for each of the target's adjacent enemies that has Pack Tactics.
So if you're surrounded by 4 creatures, each of them gets +3 to hit. If you have a shield, each of them gets only +1 to hit.
If you're surrounded by 3 creatures, and all of them have Pack Tactics, they each get +4 to hit (+1 for each other adjacent enemy, +1 for each other adjacent enemy with Pack Tactics).
It makes Great Weapon Fighting have some severe downsides, while buffing Two-Weapon Fighting and Sword-and-Board Fighting more preferable, while also encouraging your allies to engage in melee combat (which helps divide damage evenly across the party, which is optimal for 5e).
This cuts down on circling (which is easy, as you've mentioned), but still provides incentive for engaging and surrounding a solitary target, or bottlenecking enemies. Lastly, it provides a well-needed boon to melee combatants, as Ranged combat is superior in almost every way.