This is never going to happen, but I figured I'd post the idea anyways just to get it out of my head =P
The basis: Classes are removed from the game.
-At character creation, an additional tab requires you to pick three things: An armor type, a weapon type, and a magic line. When you first get into the game, these are the three skill trees you have access to.
-As you play the game, other skill trees are accessed in various ways.
-Most or all of the current class-specific skills in the game are moved into these new skill trees.
Skill lines you can choose at the start of the game:
Weapon
One-Handed + Shield (Handaxe, Sword, Mace, Dagger)
Dual Wield (Handaxes, Swords, Maces, Daggers)
Two-Handed (Greatsword, Battleaxe, Maul)
Polearm (Spear, Poleaxe, Halberd, Battle-Staff)
Bow
Crossbow
Staff (Fire, Ice, Shock) (Note: there is no longer a restoration staff skill line)
Armor
Light Armor (note: light armor no longer gives any armor value, making it far weaker than the other armor types when taking physical damage)
Medium Armor
Heavy Armor (note: heavy armor now provides twice it's current armor value, making it far more powerful against physical damage)
Magic
Destruction - Fire
Destruction - Frost
Destruction - Shock
Illusion
Restoration
Conjuration
Alteration
This is your starting setup, and gives you a pretty good variety in creating a character build right as you enter the game.
The following skill lines would be accessible throughout the game, by varying means:
Guilds
Fighter's Guild - Joined in towns
Mage's Guild - Joined in towns, cannot be used at the same time as the necromancer line
The Undaunted - Joined in towns
Werewolf - Joined via affliction/questline, cannot be used at the same time as the vampire, dawnguard, or vigilants of stendarr lines
Vampire - Joined via affliction/questline, cannot be used at the same time as the werewolf, dawnguard, or vigilants of stendarr lines
Necromancer - joined via questline, cannot be used at the same time as the mage's guild line
Thieves' Guild - Joined via quest line
Dark Brotherhood - Joined via quest line
The Dawnguard - Joined via quest line, cannot be used at the same time as the vampire or werewolf lines
Vigilants of Stendarr - Joined via quest line, cannot be used at the same time as the vampire or werewolf lines
The Greybeards - Joined via quest line
Descriptions for the abilities in each line (for new lines, at least):
Polearm
-Alternative tanking weapon line, includes a taunt and increased defenses vs multiple attackers via passives
-Spear sub-weapon type gains increased critical chance and critical damage
-Poleaxe sub-weapon type causes bleed
-Halberd sub-weapon type gains increased damage and armor penetration
-Battle-staff Sub-weapon type increases spell power and spell resistance
-Skill line focused largely on sweeping area attacks with knockdown and knockback effects
-base 10 meter range, over the 7 meter range used by two handed weapons or the 5 meter range used by one handed weapons
Crossbow
-Alternative stamina-based ranged weapon line
-No AoE attacks like the bow line
-Increased armor penetration and single-target damage over the bow line
-Shorter range than the bow line at 20 meters max for all abilities and basic attacks
Destruction - Fire
-Contains most of the fire-based abilities from the dragonknight skill trees, as well as things like a basic fireball attack
Destruction - Frost
-Frost based spells like the currently npc-only ice wall, frost atronach, ice shard, etc.
Destruction - Shock
-Contains most of the lightning-based abilities from the sorceror skill trees
Illusion
-Contains most of the stealth/invisibility and fear skills from the nightblade shadow and siphoning trees
Restoration
-Contains skills from the templar restoring light tree and restoration staff tree to create a universal healing line
Conjuration
-Contains most of the skills from the sorceror conjuring tree, including pet daedra, bound armor, and new conjuration spells
Alteration
-Contains a non-lightning form of bolt escape, conjured ward, buffing abilities like blessing of protection, and physical spells from the dragonknight trees such as stonefist and petrify.
Necromancer
-Contains life draining/leeching skills from the nightblade skill lines, as well as skeletal summoning abilities.
Thieves Guild
-Contains improvements to stealth, camouflage, lockpicking, and mercantile (vendor buying/selling prices)
Dark Brotherhood
-Contains most of the damaging and debuffing skills reminiscent of the nightblade's assassination skill line
The Dawnguard
-Contains sun-based spells from the templar's dawn's wrath skill line
Vigilants of Stendarr
-Contains aedric spells from the templar's aedric spear skill line
The Greybeards
-Th'uum / power of the voice skills based on the dragonknight's draconic power skill line as well as new abilities reminiscent of the shouts used in skyrim, though of limited power
Given a system this open, with no class restrictions, you'd finally have that 'play as you want' gameplay that ZOS keeps trying to stress. Each individual skill would be balanced against universal variables (so AoE skills would all do similar levels of damage for similar cost, or crowd control skills would have similar durations and cast times), so a player can choose based on the effect or aesthetic he wants instead of which skill is most powerful. If you want to be a crossbow-wielding witchhunter style character using illusion and destruction magic, so be it. If you want to be a heavy armor, greatsword-wielding necromancer with skeletal minions watching your back, you can do that too.
Again, this will never happen, but it's nice to throw the idea out there =P
PS4 / NA
M'asad - Khajiit Nightblade - Healer
Pakhet - Khajiit Dragonknight - Tank
Raksha - Khajiit Sorcerer - Stamina DPS
Bastet - Khajiit Templar - Healer
Leonin - Khajiit Warden - Tank