Remember the Lock quest is purely cosmetic, so changes shouldn't be anything that would change combat - ie nothing that is a dps increase etc.
Druids should do an epic quest that involves all 4 talent trees (you get free respecs for each part of the quest) and at the end of it you get a shiny new spell that lets you pick what animal you turn into. Has to be from a similar kind of tree - much like how hunters have tenacity and ferocious pets etc - so guardians could be any traditional 'tank' animal - turtles or crabs etc. Ferals could be any traditional dps animal - cats, wolves etc, not sure what would work for boomkins or trees - perhaps they could just pick different colours schemes? like trees could pick if they want to be a summer tree or autumn tree etc.
Rogues should get more 'flashy' animation, nothing big and crazy, but just more shadowy auras and details during combat, daggers dripping poison replacing enchants (that someone suggested earlier) does also sound pretty cool. Rogues are kind of a hard one because they are supposed to be a subtle class...
Hunters are another hard one, I'm struggling to think of anything thats not game changing. Possibly rather than have it as a drop quest like the warlocks had, this suggestion would be a quest picked up say at max level in the next expansion, and unlocks dual weilding pistols/crossbows, obviously balanced so that it is balanced dps between DW and 2H ranged weapons
Shamans would get more elemental flashiness going on, more lightning and rocks floating around them etc.
Death Knights, would probably just be some sort of aura or ground effect, something like desecration or defile - not damaging just cosmetic?
Warriors could get a permanent size buff - not really chaning their hit box etc (so not game changing) just look bigger?
Monks would get walking animations, brewmasters would splash beer as they walked, mist weavers would have mist trailing behind them, and WW would have wind blowing past them.
Paladins and Warlocks would both get flying mount quests.
Priests would get to switch between holy and shadow animations (so shadow priests could use holy animations and holy/disc could use shadow animations)
Mages would get alternate spells so that a frost mage only uses frost animations, a fire mage only uses fire animations and arcane only uses arcane animations - so even if you're using frost nova as a fire mage, it will appear as a fire animation or something (one thing that bugs me about my mage is that I play as frost - always have and will - but I dislike that some spells are fire or arcane)