One of my favorite class quests in the game would be the Green fire warlock quest, hell I am even tempted to level another warlock just to redo the quest chain sometimes. The mage tower was also one of my favorite quests/scenarios that gave us cosmetics. It would be amazing to see some of these return, for example a Balance druid could gain the ability to make their magic look like drust magic after discovering a druid using strange magic eventually leading up to the main battle. You could do for a mage a questline for blue fire with a questline that involves working with the blue dragonflight. The list goes on and on about possible cosmetics for abilities. I know that it was said that the Mage tower was a expansion exclusive feature but it was a ton of fun for me personally.
This would be a fun way to add visual diversity to the game. Especially if there were multiple ways to discover the quest chain.
There was a lot of talk in the months leading up to Blizzcon last year about “class skins”, thanks to some Youtuber. It was basically positing the idea that a new sort of glyph system could be added to re-flavor classes, like making the tauren paladins more distinctive or making the various priest races look closer to their actual faiths rather than generic “light and shadow”.
It doesn’t seem to have made it into Shadowlands from what we’ve seen so far, but it’s always possible that they’ve still got something like it behind the scenes. They do seem to be pulling out all the stops for customization 
Make them all like thematic as well around spec. Like for priests they can have their character radiate light or shadow on a different level based on spec, then disc is kind of a swirling mix of both.
Or maybe make them all spell animations but for some classes that’d be kind of difficult.
Also runs into the issue of some classes will be like really cool thematically. Like monks being able to be a disciple of one of the Pandaria gods and based on selection it gave them colorations to all of their stuff. Like Xuen makes blue lightning radiate off their attacks.
Then you’d have outlaw rogue which would make no sense. Would like rogues to get a cosmetic sash that has a bunch of poison vials or something though.
still waiting for the blue/pink fire spells for mages
and purple flames for destro warlock using shadow/void
That’s how you know it will never happen.
If they did do it, they’d probably lock it behind a half dozen rep grinds.
Yea I can see how some classes would have the issue of not being as cool because of their theme but who knows what blizz could come up with?
I’d love that. Honestly, more class quests. I enjoyed doing the Class Hall stuff.
Yep class hall stuff was fun for me as well…Sad they didn’t return in BFA though but I get why.