Extra Attack at level three. This is terrible, terrible balance in a game where Fighters get that feature at level five! Not to mention the Doctor gets this and an equivalent of Signature Spell at level three! No no no no!
There is an ability at level two that poisons enemies within 10 feet for 10 minutes, no save. The poisoned condition is extremely powerful, and poisoning without a save is ridiculous! A grappling Plague Doctor can lock down an entire encounter by themselves!
Poisons are nonmagical, so they can't be disrupted by anything that counters magic. I can accept this, almost, but why can they also mimic the Magic Weapon spell? How can a nonmagical substance overcome resistance to nonmagical substances? Most poisons are just reskinned or slightly modified spells, anyway. Nothing really new here.
How can this be fixed?
- Make the subclass progression in line with other subclasses. Try 3, 6, 10, 14 and maybe 18. Extra Attack at six is great! Rework the Malpracticioner entirely; right now, it's extremely overpowered and broken.
- Use format similar to the PHB. The tools are out there, and they're free!
- Bitter Humors should take an action and have a limit between rests. Maybe treat it like Channel Divinity?
- Either come up with something new or use what already exists. A "poison" that perfectly replicates a spell is just a reskinned spell, not one of forty four new spell/poisons.
- Emergency aid is lame and late. Do something new, fun, and different!
All in all, an overpowered class with little to offer. Has potential for being neat, but needs a lot of work!