I think the idea of a ranking would be good to define a role in a group and to make sure every class has their moment to shine.
Let's say we have a "tankyness" rating like this (and you obviously don't have to agree with my ranking at all - it is just how I see the classes we currently having matching up):
1. Fighters and Paladins are the classes you want to have when it comes to tanking an endgame raid boss on the highest difficulty.
2. Barbarians, Monks and Druids can also build tanky and will be able to survive pretty well but not enough to take on the strongestt endgame bosses.
3. Rangers,Clerics and Favored Souls are able to stand against most enemies without worrying too much about being killed imediatly but will have to split in other classes if they want to survive an extended fight or more than one enemy.
4. Rogues , Wizards and Sorcerers generally have to be very careful and want to make sure that they either stay away from the enemy or that the enemy isn't attacking them.
Not really sure where I would rank Bards and Artificers but I would put them in the lowest tier, just from my personal experience.
Of course this doesn't mean, if you are playing an arcane spellcaster as an example, that you end up with no options for defense at all but it means that they generally have to invest more to make it work. Also I am thinking here about what options the classes have available and just playing a Fighter or Paladin wouldn't mean you are automatically a good tank but it means they end up on top if they want to invest.
The same kind of ranking we can make for DPS(sustained and burst?), healing, CC and "utility" or "group related"(stuff like trap disabling, monk finishing moves, buffs,...) and I think it could be very productive, because then we can compare the ranking to how classes actually perform in the game and find out what classes are too weak and what classes are to strong in a category.
That being said I like your "vision" of the assassin but it would be great to have it as an intro to thread and I would actually love to know what your vision if for all the other classes and enhancent trees.