Class Orders are something that I think players had always been interested in. There's been something kind of like them from the start, but only for a few classes. Druids had an entire zone to themselves, Moonglade. Rogues had Ravenholdt, and there were a few organizations that were mostly affiliated with certain classes, like the Argent Dawn for Paladins.
When Death Knights were introduced, they got a very clear class identity, thanks in large part to their special introductory quest chain (it wouldn't have made sense to have them show up at the standard racial kiddie pools with a bunch of level 1 characters.) Death Knights actually introduced with them the real prototype for the class hall: Acherus, the Ebon Hold.
Not only was this where Death Knights began their adventures, but it became a home-away-from-home. While Druids would go to Moonglade on occasion for specific quests, Acherus gave you a reason to keep going back, namely Runeforges. It struck a very similar balance to how class halls work in Legion - you wouldn't spend all your time there, but you'd want to visit any time you got a new weapon or wanted a few smacks at a target dummy without having to go back to one of the old capitals.
And its design was such that it was just big enough to feel like a real, significant location without feeling so vast that it was a pain to get around.
Monks, the only non-hero class introduced after the game came out, got something very similar with the Peak of Serenity in Kun-Lai Summit. This not only allowed Monks to get to Pandaria long before level 85, but also gave them their own little area to chill and be Monks. Without the Runeforging mechanic, Monk players instead got a class quest every 10 levels that sent them there and granted a nice blue item, as well as a daily quest that would give them a little XP boost.
If we count Peak of Serenity, Ravenholdt, and Moonglade as class halls before that was an official thing, it's kind of interesting that Acherus is the only one to have carried over into Legion. Druids have the Dreamgrove now, Rogues have the Hall of Shadows (which is, ironically, an area of the Underbelly that was open to all players in Wrath,) and Monks have, for very clear story reasons, a new home on the Wandering Isle (which also has the added benefit of actually letting players return to that zone. Wonder if they'll do something like that for Kezan or the Lost Isles at some point?)
Blizzard often talks about features for expansions versus evergreen features. They've made it clear that artifact weapons are of the former category - we're going to have to check our Ashbringers, Doomhammers, and Scepters of Sargeras before we move on to expansion seven. But class orders, and potentially their halls, seem like the sort of thing that ought to be evergreen content.
To be sure: while the Horde and Alliance are largely sitting this fight out (we'll be charitable and say that they're overwhelmed by invasions at home,) I expect us to get our teams blue and red back in working order next expansion.
But unless we manage to seriously disgrace ourselves somehow, our Paladins will presumably still be Highlord of the Silver Hand, and our Hunters will presumably still be Huntmasters of the Unseen Path.
The key, I think, to whether class halls remain evergreen is what we'll actually have there.
The artifact altars will be pretty irrelevant when we're back to wielding relatively anonymous weapons. The scouting maps could still be relevant if Blizzard wants to keep doing these champion mission things, though I wouldn't be too upset if they didn't.
And that kind of leaves little other than aesthetics. Death Knights will still need their runeforges (ironically, this expansion they've been collecting dust given that you only have three sets of weapons that you can runeforge once and then never have to worry about again.) But beyond that, there's really nothing else to really make you go there other than hanging out in a very class-appropriate area.
Blizzard could find new ways to use class halls - I'd be happy to see them continue the theme of class campaigns as an evergreen feature - but if they don't, I think this could actually be a kind of backdoor into some of the player housing features people have wanted for ages.
One thing that's kind of fun is that you can actually collect armor appearances for stands in your class hall (Demon Hunters being the exception, as so far they've only had one tier set.) Collecting every piece of a tier set will, assuming it isn't bugged (I've definitely got every piece of Death Knight tier 10 in all three colors,) show it hanging on a little armor stand in your class hall. This kind of trophy system is something I had really hoped for in Warlords with the garrison.
But garrisons were so closely tied with Draenor - a world that we're unlikely to every make any significant return to.
Class halls, on the other hand, aren't so obviously tied to the Legion expansion. Yes, several are at the Broken Isles, but most of these feel pretty justified - the Dreamgrove is where Malfurion learned to Druid, so that's pretty important to the class. Having Mages in Dalaran makes perfect sense. And Acherus has moved once - it could move again.
And then a ton of the class halls are in areas that have little to do with the Broken Isles. Paladins' Sanctum of Light is just an expansion of Light's Hope Chapel (and a place that was referred to in the Death Knight starting quests.) The Heart of Azeroth is tied more to Cataclysm than Legion. And Dreadscar and Mardum are both other worlds.
So it would not be hard to just open new portals to these places in the new capital city area of expansion seven. It's really just a question of whether Blizzard can come up with mechanical reasons to keep them relevant.