There are lots of classes that players want. But we are talking about Demon Hunters. Players want Demon Hunters. They also want Wardens. Tinkers. Timewalkers. Bards. Rangers. Blademasters. Necromancers. Dragonsworn. Templars. Runemasters and more. Blizzard will find plenty of players for whatever new class it brings in....that trait isn't unique to Demon Hunters.
Would they fit a BL Expansion? Sure. OTOH, a major emphasis on Warlocks would do the same job. You could base Tinkers on a study of the BL technologists. Look into their history with NElfs to give Wardens. Examine the Sundering and get Timewalkers. Demon Hunters, believe it or not, are not the only class that could fit into a BL Expansion. It's not a question of if a different class would fit it; it's a question of what story Blizzard woudl choose to tell.
As for "too many"...go ahead and provide some.
IF you redesign the class from the ground up - you don't end up with WoW style DHs a la Illidan. You end up with a totally different class such as the D3 style DH. A totally pointless exercise. If you don't, you end up with a class which has a large crossover with Warlocks.
What does the DH bring that is worth that? They don't have a unique niche. They don't have a unique look or feel or theme. Anything Blizzard could implement for them gameplaywise could be added to ANY class. The one plus point they have is that players wnat them...but that can be said for ANY class Blizzard can add.
So....go ahead. List some reason to show why Blizzard should go ahead and tick off the Warlock players. I know they can do it. That's never been a question. I want to know why they should. And simply stating there are too many reasons for you to list isn't really good enough.
What would DHs bring as a standalone class that would make their impact on Warlocks worth it?