No way should Blizzard touch Demo, it's awesome right now. I don't want it changed into a tanking spec, or a melee spec. It's pretty damn fun as a pet-swarming, shape-shifting caster.
And they don't need to change it, because Demon Hunters have a wildly different approach. As I've been saying, the thematic gap between demon hunters and warlocks is quite large, and as long as it's explained in game, probably via DH trainers and mobs mocking and threatening warlocks, the differences will seep into the collective understanding of the game.
There are currently 170 warlock abilities, talents, specializations, proficiencies, and glyphs. Even if two have some overlap with demon hunters, that's two among the 100+ other things that will be created for the new class. And it's a mighty big assumption to assume that the two spells would have the same effect and hold the same prominence for a DH.
I can imagine an entire spec called Immolation, focusing on all kinds of single target and AoE fire spells. That's one thing right there we don't have in the game right now: fire-melee.
I've covered this a few times, what's one more go?DHs are a demon empowered class (just like Warlocks) who use Demon magics (just like warlocks) and summoned demons (just like warlocks) to fight demons and other enemies (just like warlocks) from both range (just like warlocks) and melee (not like warlocks but like rogues).
Demon hunters fight with stolen demonic powers. Warlocks wield assorted dark magics, some of which have a demonic source.
Warlocks ritualistically and contractually summon powerful demons to do their bidding. Demon hunters kill demons to take their power.
Demon hunters have one driving motivation: killing demons. Warlocks use their assorted dark powers to destroy whatever opposes them; sometimes that might be banishing demons, sometimes it's poisoning a nobleman who is close to exposing their secret coven.
Warlocks are casters, slinging spells from afar. Demon hunters are melee, fighting with weapons and backing it up with magic, much like shaman, paladins, death knights, and monks.
That said, we know nothing of the specs and rotations the class might use. The monk could have potentially overlapped significantly with rogues, but I don't hear that complaint now. Its asinine to say that a not yet created class would overlap with anyone else.