In this world, it really takes the quote, "No honor amongst thieves" quite literally. Assassins are more highly skilled, and possess stealth/power/precision that are beyond what thieves can accomplish. On top of that, assassins no longer require tools like lock-picks, petty trick items, etc. They carry less, but they're deadlier and (as per your complaints) much more useful in all battles, not just some of them. Also, don't hold onto the cliche look of Assassin's Creed, it's not the look of any other fantasy assassins in existence. It's an original creation by the developers. An assassin more falls in the line of this:
There is actually an Assassin's Guild that, like Elder Scrolls, seeks you out if you're qualified. Because of the level of discipline they possess, assassins follow a code of honor, even if some of them don't like it, carry less equipment, and even gain abilities that are considered borderline magic and alchemy.
Magic users, it's kinda self-explanatory what kind of abilities they gain.
Archers that gain a high enough skill can go to the respective guilds to possibly become Rogues or Hunters.
Berserkers, kinda their own class.
Wildmen/Amazons are also their own class.
Warrior fletchlings eventually can gain the rank of a full Warrior or even Demon Warrior
As stated before, Novice Tamers can become one of many different kinds of tamers, depending on the creatures they choose to make a kinship with.
And swordsmen also have a large array of possibilities: Fencers, Dragoons, Lancers, Knights, Blade Masters, Templars, Phalanxes, Holy Knights, Absent-Armors, and Fighters.