Ancalagon
Dusty Dragon
The thing is here that the balance that is important is not between your pc and the monsters - like you said, a DM could easily make make the monsters tougher- but rather between your PC and the other PCsI suggested that he buff the monsters instead, but he didn't like that idea.
I personally think the moon druid is ok, but I haven't had much experience with them at my table. I have banned other subclasses following a short campaign, so while I might not agree with your GM re the moon druid in particular, in general I do agree that they have the right to ban certain subclasses.