So I know this is a well worn line of discussion, but I have been mulling over what exactly a Class in D&D is. Its not the character's job, since in the original rules there were several races that were counted as class, (dwarf, elf, hobbit). And there is no income or stipulations around how a character behaves in their class, with the exception of the Paladin.
It's closer to being a description of a character's career- no matter what they are doing at the moment, it is a pithy explanation of what they are. A class write-up defined their powers and limitation, mechanically. So a wizard can't fight well, (or pick up a sword even) but on the other hand has a large spellbook of power they can wield. But the main thing is the ability to in one term sum up how the character works, and what they can do. Saying Vang, son of Vland is a master of the 4th circle means nothing to other players- they just want to know if he can heal or throw fireballs. So 'Class' is a kind of shorthand for how a character works.
Beyond that, the only other thing a class gets is its XP progression chart- a description of how much XP it takes them to advance their levels, and how they grow and become more powerful.
That seems like the key bit. What that chart describes is the character's destiny. It is the road of success and power that lies in front of them, assuming they persevere and don't get permanently killed.
So the idea I am mulling over is the idea that class is a bit like Nephilium, where there are certain 'great souls' that are reincarnated into mortal bodies every so often, and they are the souls of heroes. Each soul has its own unique abilities and weaknesses.
The upshot of this is that there can only be one cleric in the world at any time. there are priests, and other spiritual folk, but the "Cleric" is the only mortal alive with that particular destiny. If he is killed then some other candidate inherits the soul, and now has the path of destiny ahead of them. This means only one player can be a cleric, but another could choose to be a druid, or some other class devised for them.
Will have to think on this more...