Short version. No details, just a general guideline for each spec.
Priest
Discipline: Better shield, maybe an added, second shield with different benefits. Good preventive healing alongside PI.
Holy: Holy champion made more interesting. Remove the armor+resistence buff, it's boring. In exchange, make the choosable buffs more meaningful.
Shadow: A bit of push at slow (!) DPS (to avoid bursting in PVP). Hard to point out but I feel like they are missing one spell from their rotation.
Druid
Resto: Out of combat rez + a rule change allowing HoTs to stack up to 3 times.
Feral bear: Somewhat nerf base survivability because of PVP while adding shield wall-like high CD, good survivability ability for late-game raid bosses.
Feral cat: More DPS, focusing on single target damage. Maybe increasing DPS based on bleed.
Moonkin: Better mana management, insect swarm reducing target's chance to crit. Unique utility, in-flavor if bugs buzz around your head.
Warrior
Arms: A raid utility debuff based on melee crit. Increased cleave damage by seriously buffing sweeping strikes.
Fury: No need for any help.
Prot: Increased threat modifier from shield attacks, a talent giving increased rage when being hit.
Warlock
General changes:
1. Getting 2 soul shards from elite mobs. Soul was so big it had to be split into two parts.
2. Changing DS felhunter bonus because if you sack a succ, improved shadow damage makes life taps bigger, improving the mana you gain anyway.
3. If you have master conjurer, soulwell should give improved healthstones.
SM/Ruin: Buffing channeling ability buffs and considering making some curses stronger with talents. TWOW has big debuff cap, it can be abused.
DS/Ruin: I think they are fine as they are.
SM/DS: Would require too many changes to be good, will ignore them.
PVP specs: I am unsure.
Mage
Arcane: TWOW did a perfect job. They are a great utility class now.
Fire mage: Perfect endgame but kinda niche before AQ40. A talent reducing the target's chance to resist fire spells could make them feel better in MC/BWL (earlier, in general).
Frost mage: Would make some of their kiting/freezing spells less effective by default (mostly for PVP and aoe farm fairness) but would give them 1 big CD very good freeze CC which makes it reasonable to see 1-2 frost mages even in Naxx. Just theorycrafting, but imagine a 10 min CD freeze time effect which stuns every enemy in a 20 yards radius for 3 sec (+cold snap of course). Would give the targets a buff preventing them to be stunned by the same effect for 5 minutes, preventing abuse. Similarly how bloodlust worked in later iterations of WOW.
Hunter
Beast mastery: Further increasing pet avoidance (damage taken from aoe) and allowing some damage scaling of pets based on your weapon to make them used (specially by BM hunters) in raids.
Marksmanship: In a perfect spot.
Survival: Give them, and only them the option to lay down traps during fight + give them a unique trap. Would make them a very unique utility class. Also, some added damage to raptor strike.
Paladin
Holy: Nerf illumination but increase flash of light base healing a bit. I also feel that they lack 1 more useful single target heal. Maybe some form of shield or a heal which is guaranteed to crit if the target is below 30% HP. Something like this.
Retri: Make holy strike mostly a melee buff. In exchange give retri a built in threat-reduction and raise their melee output.
Prot: Make ardent defender useful, it should give a real sturdiness boost for prot palas. Also, give them a way to range pull properly. It's not class diversity to not give them range pull because they are just forced to take engineering and toss out bombs anyway.
Rogue
General changes:
1. Add 1-2 chests to each raid which can only be opened by 3 max-level lockpickers together, incentivizing to take at least 3 rogues. Chests contain trash loot from the raid. Helps gearing, also giving rogues a very good non-combat utility.
2. Make 'curse of nature' a poison which can be applied by rogues.
I think these two additions should be enough. If rogues really start to fall back compared to elite DPSes, considering a slight DPS increase based on DOTs to prevent PVP burst.
Shaman
Elemental: Needs better mana management, in-talent threat reuction and a way to make targets less probable to resist nature spells. They can't really switch to any other school of magic.
Enh DPS: Half of their problems would be solved by moving key enh talents into the enh tree (nature's guidance, elemental devastation). Other than that, a mana regen talent based on melee crit, a melee haste talent and +weapon skill for 2h maces.
Enh tank: Lacks way too many things to be a real thing. Adding a next level of earth shock with same damage but with + taunt wouldn't hurt anyone but would make shaman tank fans happy. Also, increasing stoneclaw totem's threat would work the same way. No harm + some fun for some weird people.
Resto: Mostly in a good place. Mana tide feels kinda useless most of the time though. Making it 3 min CD would help.