I'm a veteran of the series: First game was Arena. From what I've seen, all the TES games were about making it more accessible and "Be who you want to be"
ARENA had a rigid class system that you had to choose at the start, and limited growth within it
Daggerfall removed the rigid class system in favor of a skill-system that allowed you to define your character from the start, and further develop it as you want as you played. It also cast out for wide-appeal by making Vampires and Werewolves playable- always popular. It also added factions to allow you to further define your character within your archtypes: Is your sneak a Thief (Thieves Guild) or Assassin (Dark Brotherhood). Is your magic-user a Wizard (Mage's Guild) or a Priest(Temples). Is your warrior a Mercenary (Fighter's Guild) or honorable Champion of Justice (Knightly Order). Somewhere in between?
Morrowind made the game more accessible by paring down the skills from two sets of three and one set of six to two sets of five, removing the "Useless" skills, and balancing the skills between the archetypes to nine per Archetype. The number of factions were reduced as well, allowing you to define your character within them. Advantages and Disadvantages ended up bundled into Race and Birthsign packages.
Oblivion made the series more accessible by cutting out redundant skills, further cleaning up magic, and consolidating skills to Seven class skills (Also the number of skills within each archetype): Armor becomes a binary choice - Do you want mobility or greater protection? "Light" armor becomes the "Medium" between Unarmored(No protection, maximum mobility) and Heavy Armor. Weapons get condensed to two skills, but the weapon selections get expanded so that you can define your play-style within your skill: Faster, lighter-hitting warriors favored Axes or Short Swords, with Maces and Longswords being favored by heavy-hitters. The game's faction system is distilled to the three archetypes and Assassins, with numerous smaller factions re-introduced to allow yourself to expand further. Also, specialized questlines separate from the guilds are added in.
Skyrim does away with classes entirely, using a Perk system to finally break free of end-game "Master of None" syndrome, and favoring the "Health, Magicka, Stamina" trio to eliminate the need of out-of-character attribute enhancements. No more "Rush to 100 Endurance" for everyone, nor a need for Warriors to focus so heavily on Agility(For Stamina and Stagger Resistance), Willpower(For Magic Resistance and Stamina), and Speed (For mobility and reaction time) to remain competent in combat. The only problem here is that the Guilds stop serving the player: There's too much story in them (Because people were screaming for better story and writing for the factions) that they no longer serve to allowing you to define your character. At least Radiant Quests allow thieves, assassins, and warriors to constantly ply their trades.


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