So what niche is left at all? Do we go with niches that are filled but aren't as filled as other niches? How novel can you get in this game? What makes something valuable in this context?
You're left with this discussion always going back into the same circles and the replies, ignoring the fact Blizzard will either never make demon hunters or they will create a reason to make them that is adequately fun and new in their opinion. We're left with disputing all you like about how nothing can do that without making them into something they aren't I guess.
Nothing new to this game will bring more than anything else but what we already have when you get down to it at the most basic level. In that foundational context, we never needed or got any value out of Deathknights and we never needed or got value out of monks. We already have melee classes, all they did was put variables on preexisting roles using distinct atmosphere and flavor. Now you're left with marking your arbitrary sense of value for those variables which make a flavor worthy of value.
In the way a Deathknight or a Brewmaster or a Demonhunter all would and could bring a lot of atmospheric value, a tinker would be pretty damn incredible to see in action. It would be a weird melee ranged hybrid I guess, nothing new there but the ratio of melee to ranged, but the charm would be in watching incredible whimsical animations, lock on missile effects and sound FX, mechanical fists on springs and other gadgets, a steam punk mech with any number of duct taped animals, plants, or minerals incorporated into their abilities and buffs. Bombs being tossed, beams of light focused through lenses and beamed through refining gems locking on like targeting lasers, missile clusters. Gnomeregan bosses on PCP. Accidental chicken canons.
No matter what, you're basically either a variation of a warrior, a hunter, a mage, or priest. And beyond the aesthetic visual theme, there's not much difference between an archer and a spellcaster with ranged dps beyond visual flavor and resource mechanics. Monks went with a weird hand to hand blend with neat custom kung fu animations which made them more than a melee class, hybridized with totem like healing abilities. For years I said that is exactly what they should do with Pandaren after I saw the Pandaren companion pet break into his kung fu. It was obvious to me there was a new class niche waiting to be realized with those animations alone.
Tinkers are about the most novel direction to go in some ways next to what we've gotten so far. But they don't have much traction concerning fan demand. People have overwhelmingly wanted 3 things since WOW's beta. Deathknights, Demon Hunters, and Pandaren Brewmasters. We've almost gotten them all, almost.
Now everyone wants to make some weird ass class they pull out of their ass that has nothing to do with the hero classes we talked about in beta from Warcraft 3 in the first place. Some weird wizard or knight or something dealing with titans or dragon aspects, but other than fun flavor mechanics and atmosphere, it's all just melee dps, ranged dps, tanking, or healing. Mixed up and hybridized with a new resource mechanic game.
So what offers value to the game if you don't give a fuck about lore or the iconic atmosphere of Warcraft 3? It's certainly not another tank, another ranged dps, or another melee class. So new new classes ever?
The trick is always going to be in making the way these things melee, range, tank, and heal feel new and different.
The only place to go is in new resource mechanics games and new flavor. Imagine a warrior that fights with the tone and the experience of a Barbarian from Diablo 3. That's what a warrior should feel like, a vicious howling berserker smashing the ground and liquefying hordes of his enemies while screaming like a a mad man in leaps and bounds and tornadoes of carnage. We sort of have that, but it's a neutered reflection in our current Warrior in WOW.
But things like that are going to spice up new classes, and things like that are being ignored here for the most part by the naysayers.
New movement mechanics are another area to be explored, and that's something a demon hunter would be perfect for.