I am a bit disappointed by the lack of tactical positioning in 5e, as evidenced in questions such as Are melee combatants limited to standing around saying "I attack"?.
I have tried porting flanking from 3.5e into 5e, but I was not happy with the results. Therefore, I decided to use a facing house rule, partially based on the suggestions in the 5e DMG p.252.
Facing house rule
Each creature chooses one corner or side that they face.
Foes in red squares have advantageous facing to the creature in the green square. Attacking (melee, ranged) and casting spells with advantageous facing applies facing modifiers.
- -2 to green AC vs red
- -2 to green DEX saves vs red
Turning and moving
Turning is changing the facing by more than one tick.
- Turning after moving at least one square is free.
- Turning without moving costs 1 square of movement.
- Difficult terrain and other modifiers increase the cost as usual.
- If green moves backwards into red squares, without turning, it costs +1 square.
Green provokes an opportunity attack
- …by red, when green enters or leaves a square that is threatened.
- …when green leaves a square that is threatened by a foe that turned red by the movement.
Green does not provoke an opportunity attack
- …by white, when green enters or leaves a square that is threatened.
Creatures in red squares do not provoke opportunity attacks from the creature in the green square.
- Green generally cannot target red squares
- Area of effect spells can extend into the red area if targeted on a white or green square/corner.
- For facing modifier considerations, area of effect spells use the point of origin as shown in PHB p204, facing from the point of origin to the boundary.
- Critical hits from red squares against green deal an additional die in damage.
- Creatures in red squares do not get disadvantage for close ranged attacks.
- A shield offers no protection against attacks from red squares.
- Green Wisdom (Perception) against red Dexterity (Stealth) checks may have disadvantage.
- Some monsters (mainly non-humanoids such as beholders) may have different facing rules.
My goal is to make tactical positioning more important in 5e. However, an equally important goal is that my changes do not favour certain classes far more than others.
I did add the DEX penalty for this reason: Blasting spells should work similarly to physical attacks, and a well positioned blaster should get similar benefits.
On a side note, the rules should also be clear and fast, which is why the delineation between the red and white squares is a straight line over the battlefield.
What parts of my house rules are unfair in the sense that they benefit/hurt only specific classes in a non-trivial way? I am willing to allow minor injustices, but they should not nerf a character concept or optimisation strategy into worthlessness.
A good answer would contain a mechanical explanation of which specific changes are bad for what classes. I am not looking for general discussions of tactical positioning or flanking. An answer that shows that facing is incompatible in general with my fairness goal however would be good.