So I was thinking..
Suppose in the next expac, when we hit max level, 95 or 100, instead of a whole new talent tier added, the final tier would be a single slot where you are given an interface to make ONE customizable spell. In this interface you are able to pic the icon for the spell, and make the name, permitting blizzards censorship restrictions. This spell can also be reset and remade from scratch, extinguishing all locked Spell Runes etc.
Now upon reaching max level and having this feature unlocked you have to:
1. Do a prerequisite quest called "Spellbinding" or something, where you do a solo scenario, not too difficult.
2. Once you obtain a passive ability called "Spellbinding" from completing the scenario and quest, you are able to start molding your customizable spell.
3. Now, every class will have 3 slots for the spell, a Power, Utility, and Support slot. Now, these slots are optional, BUT you must pick at least one. The more slots you use, there is a longer cool down/more resource (mana/energy/rage) cost.
3a) These slots that can be filled with "Spell Runes"
What are Spell Runes? A new profession called Spell-weaving, or maybe a new branch of Inscription, would be added. They create class specific Spell Runes. They also are usable based off of your role (DPS, Tank, Healer).
Now say I am a warrior and I want to fill out my Power slot with a spell rune, I would go to the auction house and look for Power Spell Runes.
An example one would be;
[Power Spell Rune of Annihilation] ; Strike your opponent with reckless abandon, dealing 185% weapon damage.
Now that I have my Power slot filled with that, I will opt to fill my Utility slot now
[Utility Spell Rune of No Escape]; Target's movement speed reduced by 50% for 10 sec.
or [Utility Spell Rune of Ferocious Power] Add's + 5420 damage to 185% weapon damage.
By choosing to add this slot, my rage cost is increased from 20 rage to 35.
Now I am going to add a Support slot.
[Support Spell Rune of Maniacal Rage]; 20% Chance to enrage you.
or it could heal,
[Support Spell Rune of Flesh Wounds] Heal yourself for 15% of your hp over 3 sec.
This will increase the spell's cooldown from 10 to 15 sec for the maniacal rage rune.
However, picking the Flesh Wound rune it increases from 10 sec to 45 sec.
The final spells tooltip would be; "Strike your opponent with reckless abandon, dealing 185% weapon damage (+5420 If you choose Ferocious Power), slowing them by 50% for 10 sec. Has a 20% chance to enrage you."
This is purely an example of how it could be played out, and I disregarded the spells a warrior currently have, this is just an EXAMPLE. It could add more diversity to each person based on their play style I think and if Blizz were to approach it with adding TONS of spell runes it would be pretty cool I think. Obviously there would be many balancing issues to be had, plus GCD problems etc. Post your opinions or ideas!
edit 1 - For balancing issues blizz could;
"You could use a polygon or pentagram or w/e, if you remember the stats distributions from the old pokemon games. Basically spending a certain amount of a certain value allows you less of another one. It would be essentially like item level but for a spell." - Raybourne