Kurotowa
Legend
The UA8 Monk is a little different because a lot of it depends on an active defense; using their reaction on Deflect Attack. This makes them very good in a 1v1, even against a boss, where they're not likely to get hit more than once or twice a round and can mitigate a lot of the damage. Meanwhile a minion swarm can overwhelm them, when someone with more passive defenses (high AC, Barbarian damage resistance) can weather the minion swarm more easily.I think the D&D monk can fit a similar role. Primarily being a damage dealer, but in times of need they have the ability to move around the battlefield better than most. They can go from being front of the pack, punching the boss in the head one round, and then next round slip to the back of the group to help protect the wizard from the wave of minions that just showed up.
It's a bit of a new niche, but that's why I think it may work. There's not a lot of direct competition for it. And thematically it kind of works, that the martial artists excel at single combat but can get overwhelmed by numbers in a way a more battlefield focused warrior wouldn't.