Right after Demon Hunters, the biggest feature of Legion is the idea of "Artifact Weapons." Rather than getting new weapons as the expansion goes on, we'll instead get new relics to plug into our existing weapons to change the stats. This aspect of artifacts could actually wind up being pretty much how it works now, but with the new idea that that really fancy weapon you acquired doesn't simply get tossed aside for a better one.
Interestingly, Demon Hunters are going to be getting a unique weapon type - Glaives (the existing Warglaives of Azzinoth will remain swords so that other classes can use them - DHs will be able to use Glaives and One-handed Swords.) The Warglaives iconically wielded by Demon Hunters have always been kind of unusual - literal double-edged swords (but with a handle in the middle, of course.) You could just make them technically swords, and I actually think Blizzard might consider simply doing that when Legion actually comes out.
But it does sort of raise an interesting possibility. When Death Knights were first announced, Blizzard had considered giving them a unique weapon type - the Runeblade. In fact, they seemed to toy with the idea of a kind of early version of this artifact weapon concept back then - coming up with the idea that maybe a Death Knight would keep a single weapon (or pair of weapons) forever, but upgrade it as they went along.
In the end, Death Knights basically make their swords and axes into runeblades through runeforging (and their maces into runemaces?)
But the acquisition of weapons these days is a lot more sort of guided than it was then. With personal loot (and granted, in guild raids, you'll probably still be using master looter,) you can easily put in items that only one class, or even one spec would want, without worrying about crowding the loot table with stuff that will just get sharded. Already, since Mists, every Gun, Bow, and Crossbow has been exclusively a Hunter weapon (and soon it will be exclusively a Marksmanship/Beast Mastery weapon.) Agility Daggers, likewise, have been just for two Rogue specs.
After Legion, when we're probably going to be going back to normal weapons (artifacts seem more like a one-expansion feature, as opposed to an "evergreen" feature, though that depends on how successful it is,) there will have to be some thought given to weapon types and weapon acquisition, but Legion presents Blizzard with the opportunity to go kind of nuts with weapons. Every weapon will be designed specifically for its respective spec. They could try out this idea of weapon types exclusive to certain classes - perhaps even using the Diablo III system, where there are certain types of weapons that are exclusive to particular classes, but every class can also use the generic types. (As an example, only Crusaders can wield Flails or Crusader Shields, but they can also use regular shields and maces, swords, daggers, axes, spears, etc.)
Glaives could open the door to new weapon types. Here are a few that I'd be happy to see, for specific classes:
Kegs: Monks - I'd really like to see my Monk carrying a big old keg of ale on his back - perhaps even using it as part of the Keg Smash animation.
Hammers: Paladins - it's really frustrating to me that there are so few good warhammer/maul models in-game. Azeroth's Paladins are known for their iconic hammers (see the animations for Judgment, Templar's Verdict, Final Verdict, Execution Sentence, Light's Hammer, Hammer of Wrath, Divine Storm, the freaking class icon...) but pretty much after vanilla or maybe BC, there have been roughly zero really good blunt hammer models for Paladins to use - everything's covered in spikes instead. I'm stuck using my Hammer of the Naaru (ok, it's technically a Sunsoul Warmaul, but same model) which is great, but my Human feels like he's appropriating Draenei culture. Maybe the key is to separate out Maces (available to practically everyone) and Hammers (Paladins only.)
Totems: Shaman - ok, that big old totem that Cairne used (and yes, he was a Warrior, shut up) would be really cool for a Shaman to use. Alternatively, taking the concept of the Witch Doctor's Ritual Knife (though to be fair, that's pretty close to a dagger anyway.)
Spears: Warriors - You know, we have polearms, which pretty much have spears, halberds, and the real-world version of glaives under their umbrella, but I'd like to see Protection Warriors using one-handed spears, Ancient Greek Phalanx-style!
Tomes: Warlocks or Mages - Books often get use as off-hand items, but sadly, they never really get seen. I'd like to see a Warlock pull out a tome of unspeakable knowledge when they're casting something like Chaos Bolt. Though this might just open us to a larger conversation about whether casters should use their weapons as part of their casting animations.
To be fair, there might not really be a concept like this for every class. But it might be fun to free up some design space from the existing weapons. I'd be really cool to see a Combat Rogue using a whip or something, or letting Paladins get a bit of that Crusader flavor and letting them swing around a flail (though a flail seems like it would be terrible for tanking - how do you parry with a flail?)
We'll have to see how tricky it is to add a whole new weapon type to the game (and in fairness, we'll really not see that effect until the expansion after Legion.) But if it goes well, it might open up some fun new possibilities.