As soon as Death Knights were announced for Wrath of the Lich King in 2007, speculation began on when we would get Demon Hunters as a playable class. Before Death Knights, we had gone three years (and would go a fourth) with only the original nine classes in World of Warcraft. Death Knights opened the way for new classes in WoW, but historically, they've only come every other expansion, meaning that even though there was only one class to come between Death Knights and Demon Hunters, the time difference is a whole eight years.
But of course, Monks were also a pretty worthy addition to the game, and while I don't think they've ever met the popularity of the DK or the other vanilla classes, there are certainly plenty of people who main as Monks.
Almost every expansion has created what I call the "New Toon Incentive:" a reason to create a new character. I've been playing most of the characters who I spend the most time on since vanilla - my Paladin, Hunter, Druid, Warlock, Shaman, and Rogue - the ones who are the "class captains" amongst my alts, are all characters I created in the fall of 2006 before even Burning Crusade came out.
Historically, we've gotten new races and new classes in order to get people to try something new. Back in the days before a paid race change, if you wanted to create a Blood Elf as your new main, you had to play that character from level 1 (or 55 if it was a Death Knight.) And of course, there have never been paid class changes (and probably never will be,) so at least during Mists, you really had to level up your Monk from the start.
The character boost in Warlords of Draenor was almost an anti-new character system - it took a brand new character and caught it up to the point where it wasn't at a serious disadvantage at the start of the new expansion, but it did encourage some (those who didn't already have every class at 90 - I still haven't used my Warlords boost) to try something new.
The fact that Demon Hunters automatically get to level 100 after their starting experience also sort of negates their "newness" as characters, but given that it's a brand new class with its own mechanics, and that if they didn't it would be the default move to simply use one's Legion boost on one's Demon Hunter, I can forgive that.
We are of course in an era with Timewalking Dungeons, and I hope one day we'll get Timewalking Raids as well (probably make that a pre-made group kind of activity,) so even though Demon Hunters will be skipping twelve years of WoW content, they'll be able to go back and experience some of it at the appropriate power level.
Anyway, a question emerges, which is what the next New Toon Incentive (NTI) will be.
Warlords of Draenor did not bring a new race or a new class, but that was I think largely because Mists of Pandaria brought both. I've argued many times that a new race is far easier to introduce into the game, though their story might take a back seat.
I actually think Warlords would have been a fantastic opportunity to introduce playable Ogres for the Horde and Arrakoa for the Alliance. Given how recently we've been to Draenor and honestly how much Blizzard seems ready to move on from the problems of the expansion, I don't really expect to see much focus on Outland or Draenor any time soon, which definitely affects Arrakoa, though Ogres have enough of a presence on Azeroth that I don't think we need to go back there to bring them on board.
I think Naga could make for a really interesting playable race, but there are definitely some questions about their anatomy - like how pants or boots would work on a Naga character, or how they would ride on mounts. One could imagine a sort of "devolved" Naga that retains more of its Night Elf anatomy - still being serpentine, but retaining legs. Still, if they're planning that, planting the seeds for it in Azsuna during Legion would be the smart time to do so (though I do know there are some massive Naga hulks that seem to have more crocodilian legs than serpentine tails.) The racial abilities write themselves - like Undead, they'd have underwater breathing, but they'd also get a speed boost while swimming.
One would also have to deal with whether they are fully corrupted by the Old Gods like the Black Dragonflight (before Wrathion's purge, after which the question is "what Black Dragonflight?") or if they were simply transformed by them like with the Curse of Flesh. Blizzard could easily retcon this, as the history is pretty vague to begin with.
You could also really expand on the lore of Murlocs, Troggs, Gnolls, Kobolds, or other stock hostile creatures to give them enough of a culture to play them, but that might be a little too silly.
Ethereals are another popular choice for playable race, and I actually think you could do fun things with using their bandages as a substitute for skin and facial features.
Of course, there is still the option to add new classes to the game. Here I think you have to be really careful. Death Knights and Monks were able to carve out some pretty distinct niches when they were introduced, but Demon Hunters absolutely stepped on the toes of Warlocks, requiring a serious redesign of the Demonology specialization (even though I loved the old Demo, I actually really like the new one too, so I'm not too upset - I just wish I had some ability affected by my Codex of Xerrath.) I will always be happy to see a new class, but it's going to be tricky to find a new archetype.
One archetype that is often put forth is the Tinker - a class that uses devices and technology to fight and potentially heal or tank.
The good news is that this archetype could fit pretty well in a Mail armor slot - having jangly metal armor sounds perfect for a Tinker. It would also be very different than any existing class, though it would definitely step a bit on the toes of the Engineering Profession. Is that the end of the world? I don't know.
Beyond the Tinker, I can't really think of much else. A Barbarian would be too similar to a Warrior. A Necromancer would be the blend of a Death Knight and a Warlock (actually, it would make total sense for Necromancer to be a Warlock spec that works almost exactly like Demonology does now, only summoning undead instead of demons.) D&D has Sorcerers as a distinct class from a Wizard, but both are pretty darn close to the Mage archetype.
Thankfully, Blizzard has four years before anyone expects a new class to come out, and at this point I don't think anyone would blame them too much for choosing not to add more to the ranks. I was holding out for a very long time for Demon Hunters, and now that we've got them, I'm pretty satisfied with the class selection in WoW. (Though I'd be really into a Tinker class.)
There's also the age-old idea of adding new specs. That being said, with the new changes to how changing specs works, that would hardly encourage anyone to create a new character. Ranged specs for Rogues, Demon Hunters, Paladins, Warriors, or Death Knights would all fit fine with their class concepts, and there are also some crazy ideas about making a melee Mage or even a tanking Warlock (though again, Vengeance kind of fits that concept already.)
New specs for existing classes (Druids would probably be exempt as they already have 4) could be cool, but they might wind up being even more work and trickier to balance than adding a whole new class.
New race/class combinations aren't a huge incentive for new characters - especially in the days of paid race changes, but they are very safe for game mechanics. Cataclysm saw a ton of these, and we got one new one in Legion with the addition of Gnome Hunters. A lot of us are hoping that in the future there will be other options for Demon Hunters (I still want a Worgen Demon Hunter.) Granted, things are so much more open now than they were before Cataclysm that if we go much farther, it might just come to a point where every race can be every class. Sure, your Draenei Warlock would be utterly terrifying, but would it make sense in the lore?
Beyond those options, I think some really serious changes would have to happen to encourage people to create new characters. We don't know what the expansions after Legion will be, and while I'm always down to speculate (Old Gods/Warcraft-Equivalent-of-the-Underdark???? High Seas Adventure Fighting the Naga????) I'm also pretty happy to take my Demon Hunter in the fight against the Burning Legion.