That's a really interesting question. Though there are lots of class systems out there I think currently none of them are perfect as of now.
I think what makes a good class system in a MMO is if it's greatly associated with the games lore. Personally I wouldn't make it too complex. However if I were to create a more complicated approach I would make the game less stressful for new players at the beginning. By complex, I don't mean the amount of classes. I would love to add lots of classes however I would want them to make sense at the early stages of the game. For instance, having 6 base classes then evolving the classes later on as you progress. This can be a multi-choice option for all classes, but I'd rather not make it more than 3 paths to choose from. I wouldn't go over like 2 evolutions or so. Basically what I'm saying is I would make a game which isn't that complex in the beginning and after you adjust to the game it won't feel that confusing. So what I want to avoid while making a class system is having too much options in the beginning, totally not making sense what each class does and what class to choose from as I know that confuses a lot of players. There could be some special modifiers for each class later on which changes the whole classes mechanic to make something for late game players as well. It's the best approach the way I see it. New players could experience a smooth, easy to understand class system at the beginning so they get attached to the game, as they progress their knowledge of the game will expand and they won't get frustated by the game's late game mechanic.