There are lots of topics on the forums discussing possible 4th specs for existing classes in the next expansion. Meanwhile GC tweeted that he's concerned with hunter and rogue spec diversity and stated that the devs consider combo point system (and thus various secondary resources) a better class design. In MoP mage specs have become too similar to each other, especially with their lv75 and lv90 talents. On the other hand, warlocks have been redesigned for MoP and achieved great results. I remember reading the commentary (GC's?) about MoP warlocks - that the devs like the new design and want to see how it'll play out, with its different resource systems among specs and especially rogue-like mana/energy, burning embers/combo points of Destro.
So maybe they're thinking of revamping the remaining pure classes in the next expansion, warlock-style? All mage specs could use some lift up, but the most troublesome is no doubt Arcane. Since its complete revamp in WotLK it has been reworked nearly every major patch. Judging by its popularity in MoP only due to cheesing the mechanics with Scorch-weaving and the general lack of interest to the spec after Scorch removal, I'd say the first mage spec to receive an overhaul is Arcane. So here are my thoughts on how to make it more interesting.
Instead of generic debuff, Arcane should have a proper secondary resource in Arcane Charge (AC). Maximum 4 AC, build up by Arcane Blast and Arcane Missiles, spent by other arcane spells. Consider lv75 and lv90 mage talents replaced with completely new ones.
Arcane Blast - gives 1 AC, 2 if it crits.
Arcane Charge - increases damage and mana cost of your arcane spells, maximum 4 AC.
Arcane Missiles - a proc, gives 1 AC, costs no mana, maximum 2 procs.
Arcane Barrage - has no cooldown, spends 1 AC per cast, can be cast with no AC, puts a 3 second Overwhelming Power buff on you that increases Arcane Barrage damage by 15%, stacks 3 times.
Mana Gem - no longer creates a gem in your bags, but spends all remaining AC to restore x% mana per charge.
Evocation - instantly grants 15% mana and 1 AC and also 45% mana and 3 AC over 3 seconds, 2 min cd.
Temporal Rift (new) - 2.5 second cast, creates a temporal rift to deal massive single-target damage, costs 4 AC and a lot of mana, 15 sec cooldown, no cooldown if used against a target with <20% health.
Arcane Explosion - your Arcane Barrage, Arcane Blast and Arcane Missiles create Arcane Explosion on impact (AM explodes for each of its missile waves), but cost 25% more mana, 10 second duration, 10 second cooldown.
Nether Tempest (new) - 2 second cast, deals damage to all targets in 10 yd zone over 8 seconds and applies Slow debuff, 12 second cooldown, replaces Flamestrike.
Arcane Power - instantly grants maximum Arcane Charge and keeps it at maximum for 15 seconds, you can't cast Mana Gem while Arcane Power is active.
Arcane Instability (new) - passive, reduces the mana cost of spells by 30% if they critically hit.
Arcane Repulsion (new) - your next Arcane Barrage will knockback and daze its target for 6 seconds or your next Evocation will reflect all spells while chanelled, 30 second cooldown.
Slow - Reduces target's movement speed by 50%, and increases casting time by 50% (25% on player targets) for 15 sec.
Mastery: Arcane Charge (new) - increases additional damage provided by Arcane Charge and mana per AC provided by Mana Gem.
Basic rotation - AB/AM to 4 stacks, Temporal Rift, AB/AM to 4 stacks, ABarr x4, weave in Mana Gem if low on mana - repeat
Burst rotation - Arcane Power+trinket+pot+etc, ABarr, Temporal Rift, Abarr, AM if procced, ABarr, ABarr, Evocation or Mana Gem after to restore mana - keeping ABarr buff at 3 stacks while casting TR and AM if available.
Execute rotation - AB/AM to 4 stacks, Temporal Rift.
Execute burst rotation - Arcane Power+trinket+pot+etc, Temporal Rift, AM if procced, Temporal Rift, Temporal Rift, <...>
AOE rotation - Nether Tempest, Arcane Explosion, proceed single-target rotation
Mobile rotation - ABarr, ABarr, ABarr, <...> - if you have charges, then you don't lose dps while moving, if you don't - you still do ok damage with buffed ABarr, not resorting to negligible damage of Fire Blast and Ice Lance. Mobility talent tier helps greatly with keeping some AC up or finishing movement faster.
So how do you like it? What would you alter, add, remove? How do you think mage specs could be improved in the next expansion?