I love dungeons. There was a time when dungeons were truly the meat-and-potatoes of World of Warcraft at the level cap. If you wanted to get meaningful progression done, but your guild wasn't hitting the latest raid that day, or perhaps you didn't do much in the way of raiding, running a dungeon would be a perfectly good way to make your way toward better gear.
That started to falter in Mists, especially when 5.4 did not bring any new valor gear, but it really died in Warlords of Draenor, when Justice and Valor points disappeared and all we got for running a random dungeon with gear that was useless to us was garrison resources - something you could literally generate simply by waiting, and that was used primarily to build buildings that you already probably had up or send followers on missions (many of which granted you garrison resources.)
We're at a point where a fresh 100 player could simply skip dungeons entirely, moving directly to Tanaan Jungle to get far superior gear that would qualify them very quickly for Hellfire Citadel LFR.
Blizzard has noticed this, and one of the ideas they threw into the Legion announcement was that they felt that they had not done right by dungeons, and that they plan to have dungeons remain relevant throughout the expansion.
The question is: how?
Idea 1: Just bring back Justice and Valor Points already!
JP and VP were a fantastic system to keep old dungeons relevant. It got highly-geared players to run newbies through dungeons, and it gave you a reason to keep running, while not forcing you do to so, as serious raiders could approach their weekly caps by defeating raid bosses.
This is purely one of those "if it ain't broke, don't fix it" issues that Blizzard, unfortunately, "fixed." One concern, that was admittedly linked closer to late-expansion dungeons like End Time, was that they felt it allowed players to skip over existing content (once you could get Firelands-equivalent gear from End Time, Well of Eternity, and Hour of Twilight, very few people would run that raid anymore.) But in this era of LFR, that's really not a concern. Raids are consumed at a far faster rate than they used to be, which means that you never really have to worry about players skipping past those raids if they have any interest in them. Everyone had plenty of time to clear Blackrock Foundry several times over before Hellfire Citadel came out. So while only a fraction of the playerbase had defeated Ragnaros when Dragon Soul came out, I'd suspect that a huge portion, if not the majority of players (or at least the majority of players who had any interest whatsoever in raiding) had been able to defeat Blackhand at least on LFR before Hellfire Citadel was released.
And it's not like there haven't been other catch-up mechanics introduced that just don't happen to be from dungeons. Tanaan Jungle gear can scale up to 695, which is better even than Hellfire Citadel's LFR gear. (In fact, I think the Baleful apexis gear went too far.)
Idea 2: Artifact Weapon Points
We still only have a vague idea of how Artifact weapon progression is going to work. It looks as if we'll earn points through various means (including downing dungeon bosses) to unlock more talent-like bonuses for our artifact weapons.
What I wonder, however, is how deep this system will be. It would be problematic if we could all completely fill out our trees two months into the expansion. Hopefully we'll see the feature expanded over the course of the patch releases.
Idea 3: Crafting Materials
Professions are getting a revamp in Legion, but we don't really have the details on how they're going to work differently (one thing confirmed - far fewer daily cooldowns.) Theoretically, you could kind of combine the idea of profession and valor points, having professions function as the alternate-gearing system, but you would really need to get rid of the three-crafted-pieces limit, which they say is going to stick around.
Idea 4: Lots of future dungeons
The far more radical design would be if there are actually a massive number of dungeons that are being held back for later patches. There are nine launch dungeons with Legion - an improvement over Warlords, but only bringing us back to Mists numbers. If every raid tier came out with four or five dungeons, you could imagine these feeling like a real type of progression - essentially giving us new dungeon tiers.
Still, I think this is highly unlikely, given the resources required to create new dungeons. My hope is that we'll see some new dungeons with each raid tier, but it's unlikely that we're going to see the launch dungeons completely outnumbered by patch dungeons.
Idea 5: Multiple layers of Mythic
6.2 introduced Mythic dungeons, which are tuned quite difficultly and are built on the assumption that you have a fair amount of Tanaan or raid gear. Unfortunately, the reward isn't really up to the difficulty, as any non-Warforged gear that you get out of Mythic dungeons is only on par with the stuff you're getting out of Hellfire Citadel LFR, which is significantly easier, and less powerful than stuff you get from Tanaan Jungle, which is merely time-consuming.
Still, I could imagine basically introducing a new tier of difficulty for each dungeon with each raid release. This isn't really ideal for me, though, for a number of reasons. The first is that it splits the playerbase. It's really best when you can throw everyone in together, as this reduces queue times and just generally makes the feature seem more vibrant and alive. The other is that it's not really that exciting to find yourself wiping again on a boss that you already feel you conquered, especially if you had conquered it a full year ago.