Fighter: The class is good, the subclasses are disappointing to me more often than not. Maybe that isn't a good enough reason for me to vote for it, but I've played a couple of fighters that were interesting in spite of their subclasses, not because of it, and that feels like it could be improved.
Monk: Several times I've been playing a monk and asked myself "what am I supposed to be doing". The answer often is "Doing less than others unless you spend chi points every time you can, then you have a 50/50 shot of doing what you want, or doing even less". I've played monks well and been rewarded, but I've played them merely OK and been punished, which is harsher than other classes I feel. Seems like maybe there is some room to make them a little more forgiving and flexible.
Ranger: I'm actually 90% fine with the class as a class (I've seen enough ranger's in play to know their base body will get the job done), but again, the subclasses are a mess. Different from how I see fighters, the ranger really relies on their subclass for a damage mechanism, and each class reinvents the wheel to try and make a competitive damage feature that is unique and flavorful and in the end what you get is a parade of different illustrations of how to make a janky damage mechanic no one really approves of. Seems like there could be some unification or re-tooling of how subclasses contribute to ranger's damage potential.