I think most people would agree that, for what things Legion might have gotten wrong, it absolutely NAILED the concept of “Class-fantasy”. Now, while some Class Halls were maybe received a little better than others, I think most people would still mark them a great addition overall.
For my part, I would love to see them return with a bit of importance. Not tying any sort of “power” to them, but rather, cosmetics and things that contribute to that sense of “Class-fantasy” once again.
For one, it would be great just seeing a bit more lore with them. Maybe they could have a quest sending you to the new expansion areas, giving you a little more context for things (ie. “Why the hell would my Death Knight be interested in the Dragon Isles?”), or even just moving things along (ie. maybe the Silver Hand elect a new Highlord, like introducing Verigan, a Dwarf Paladin after whom “Verigan’s Fist” was named after, allowing the Silver Hand to continue doing things in the story).
This might sound like a lot, but I’m honestly not suggesting this become some core feature of the game. Rather, making it something a little more evergreen.
Now, what I would maybe suggest is moving some of the Class Halls around. Paladins’ is perfect, imo, but I feel like Rogues should be using Ravenholdt Manor as their base of operations. Mages are obviously going to want to move for… reasons… but maybe they move the spire (perhaps even expanded with a library) to old “Dalaran Crater”, where other classes just see the crater itself, but only Mages can pass through the bubble to see the real spire that’s hidden by an illusion.
Or even just touching things up with a little TLC. As much as I’m sick of Acherus, there’s technically nothing wrong with it; maybe just add some NPC’s that with randomly banter about ongoing events, or maybe build something around the “roof” of Acherus, which feels like it’s just begging for some kind of additional use.
Point is: Class Halls were generally very cool and super flavorful. They did a lot to make your character feel “special”, and make you see classes as more than just “a slightly different rotation”. I think showing them a little extra love would go a long ways.