So first a big caveat: I haven't played my Warrior very much in Warlords. It's a shame, because he's my main Worgen character and I love the Worgen (the other is a Rogue, who is upstaged by my Undead Rogue - simultaneously my Horde main and member of my favorite Horde race.) But my dissatisfaction with Warriors this expansion has actually afforded me a pretty broad (though not deep) spectrum of Warrior experiences. I've tanked, Gladiator'd, Arms'd, and Furied in both varieties.
Rage, Rage against the Dying Light:
Warriors are the universal class - available to every single race ever since Cataclysm gave them to Blood Elves. Given how constant war is in Azeroth, it makes plenty of sense that everyone would have some sort of heavily-armored front-lines fighter.
With Legion focusing on class fantasy, that puts Warriors in a kind of strange position. Warriors actually have a very broad class identity. In a way, much as Hunters are "anyone who uses a ranged weapon" (and soon to be even more than that,) Warriors are "anyone who fights with heavy armor and big honking weapons without some kind of magical supplement to their strength." Warriors' plate brothers-and-sisters-in-arms both have a great deal of specificity to their flavor. In fact, Death Knights are all members of the same formerly-Scourge-affiliated order.
But Warriors can be anything from a rural militiaman to a decorated royal knight to a frontier berserker to a samurai-like warrior-poet.
Likewise, Warriors don't really have a unified calling. If anything, it's the Warriors of both sides who are most likely to drum up the passion to fight against each other. The closest thing to a unifying faction I could imagine would be something like the Brawler's Guild - an opportunity to show off one's prowess and demonstrate one's techniques.
With the Class Orders taking a more central role than the player factions in Legion, one wonders how the class is going to pull together.
I have some suspicions that Warriors will find their home in Stormheim. The Vrykul are kind of a fascinating balance between Orcs and Humans - if you were to blend the two races, and then give them a Gnomish World Shrinker, you'd basically have a Vrykul. The Vrykul, like the real-life Vikings who inspired them, believe in an afterlife for those who die gloriously in battle. The cause for which one fights is not what matters - it's how passionately and well you fought. That seems pretty on-point for Warriors.
And given that Warriors are governed by their passions, it might make sense for Warriors to take over a Vrykul mead hall. What better place for them than a loud hall full of booze, where you can get into friendly brawls any time you wish?
This, of course, does skew a little to the Fury side of things, but I can imagine having a quieter tea-room for the Arms Warriors, and maybe a practice-yard out back for the Prot Warriors.
In terms of class champions, the options are enormous. Roughly half of the faction leaders are Warriors, and they've got to represent probably the majority of all NPCs. I'll be sad if I don't get a drunken Dwarf, an Orc blademaster, and a Night Elf sentinel at least.
To Arms, My Brothers!
I'll come out and say this: Warlords killed Arms for me. I leveled by Orc Warrior as Arms through Wrath, and my Worgen was Arms through Cataclysm and Mists. It played wonderfully, and they utterly killed it in Warlords.
I don't know if simply rolling back the changes is what Blizzard would want to do, but I think they do need to make a decision about whether Arms is the "precise, practiced blademaster" type, or if they're the Executioner who lands massive blows. Either way, they need to come up with something more interesting than "wait for Rage" as a class mechanic. Bringing back Mists-era Arms would do just fine by me.
In Mists, they had this idea that each Warrior spec would have one ability that generated, rather than costing Rage. It seems weird that they took this active control away from Arms, of all specs.
Feel the Fury:
I've been mostly Fury this expansion. One big question is about their artifacts - whether it'll be a pair of two-handers or one-handers. To me it seems obvious that they should go for the former, given that dual-wielding two-handed weapons is such a unique and iconic feature for the spec.
In terms of mechanics, I think they could come up with more interesting ways to spend Rage. So much of the spec is built on whether you can get Enraged or not, but the only thing you really spend that Rage on (single-target, at least) is Wild Strike. It might not be broken enough to make any real changes.
The big question, I think, is if Gladiator's Resolve is sticking around. It's the only currently existing way for someone to DPS with a sword-and-board, but it's certainly a little awkward, given how many of your abilities are built around tanking. They could spin Gladiator off into a different spec, but that seems unlikely.
Warrior tanking has always had this odd thing where it used to be the only real tanking spec, and since then, many of the other tanks have kind of overtaken it in interesting design directions. I don't really know what to say about tweaking Warrior tanking. Tanking design space is, naturally, a bit more narrow, and I don't really know what you could take away without robbing them of something useful and cool.
You Wouldn't Like Me When I'm Angry:
Warriors have been, frankly, generic in WoW's lore, so it'll be really interesting to see them given something specific in Legion. One also has to wonder a bit about the imminent fate of one of Azeroth's most important Warriors, Varian Wrynn, as there are some fears that he'll fall at the Broken Shore to leave his Priestly son as King of Stormwind. This is pure speculation, of course, but even if Varian goes down, we won't exactly be light on famous Warriors to bring to bear (off the top of my head, Shandris Feathermoon, Jarod Shadowsong, Muradin Bronzebeard, Baine Bloodhoof, Genn Greymane - who is a confirmed major character - Varok Saurfang (fingers crossed) Lor'themar Theron (or is he a Hunter? Or is he a Paladin?) Point is: there are plenty of important Warriors in WoW.