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.