What effects would allowing to flank with reach have on game balance?

Question

One of my players asked me recently if I would allow their character, who has 10ft. reach, gain advantage from flanking while still using their reach.

I am aware that RAW this would not work since you have to be adjacent to the enemy to flank but I don't tend to blindly stick to RAW and refuse cool ideas if there is no good reason to. I also want to keep my players happy and they seemed quite fond of this character concept (to clarify, they are playing a different character at the moment but are not completely satisfied and have been thinking of introducing a different one).

What would be the effects on the overall balance if I introduced a house rule allowing to flank at reach? If I were to do it, the rule would apply to PCs as well as to enemies. Are there any good reasons not to do it, would it break anything, allowing any obvious cheesing or anything of the sort?

I've spent some time thinking about it and I think that as long as the players are aware this could be a double-edged sword, not just a buff for one of them, things will remain balanced but I acknowledge that rules on reach aren't ones that I use often so I could be missing things. I am interested in keeping the kind of balance between the party and the monsters, without giving a substantial advantage to each side, but also between party members themselves and none apart from this single character currently use reach weapons.

In case that matters, the rest of the party is made up of a hexblade warlock, bladesinger wizard and light cleric. We play on a grid, with 5ft. squares.

Answer 1

It's not quite harmless

In general, allowing creatures with reach to flank will strengthen the characters and make the game a little easier for them. The reason is that the characters will have access to this in every fight consistently. Among monsters on the other hand, reach is pretty rare, and rarer still on those that tend to show up in large numbers and would most benefit from flanking. Not even a large Ogre has reach any more in 5e (while they had it in 3/3.5). Because the characters get to enjoy this all the time, and the monsters only occasionally, it is a little unbalanced in favor of the characters.

Also note that you can make builds that profit in other ways from larger reach. For example, characters with the Polearm Master feat and a polearm will be able to attack anybody entering their larger reach in addition to being able to flank.

Will it break anything? Maybe not, but Polearm Master/Great Weapon Master builds are already amongst the highest melee damage dealers in the game. Giving them an easy, consistant source of advantage that negates the -5 penalty to hit for getting +10 on damage is going to make them even better. My own experiences with such builds that outshine all others on damage output is that they do not make the game more fun, as they warp the behaviors of the rest of the party to further support this superior strategy, and they make it harder for the DM to create balanced encounters that are a challenge to all characters.

I think you stated elsewhere your players consider you a very generous DM. This will likely not burden the game to a point where you cannot rebalance it by making adjustments to the encounters, but I think it will create a little work for you, if your players exploit it systematically