I personally would love to get a cosmetic adjustment for every class with a specific class feel to it. Make it somewhat like the legendary cloak quest line in the sense that everyone does the quest but the rewards are a little different. Not quite that involved, but somewhat like the warlock where you have to get a drop and then complete a series of difficult quests.
I would love to see DK eyes changed from just a glowing blue to have the LK "mist" that oozes out the sides.
I would love to see the priest halo.
I would love to see warriors get a battle tattoo of some sort.
I would love to see paladins get a righteous glow effect on their hands.
Make it a cosmetic thing that can be turned on and off with a UI checkbox and that is subtle enough to not clutter the screen during combat but noticeable enough that you can tell they did that questline when you are standing around waiting to pull the next boss or buff in the starting area of a PvP match.
Unfortunately Blizzard as a company has no vision and loves to say "sure we have tons of money but you can't just throw money at development!" This is one of those examples where there should ALREADY be tons of cosmetic stuff to do in the game that has no power benefit and is something that you could literally just throw workers at to get it done. It has not been here for 9 years, what makes you think it will come now at the end of the game's effective life?