On one hand, you're saying Demon Hunters don't have enough of a niche to fit anymore - which I can kinda understand, I fully agree that classes / specs shouldn't be added if they step on the toes of other classes. As much as I'd like to see a necromancer, I think at this point it would step on the toes of warlocks more than is safe.
On the other, you're saying that you seem open to giving other classes a 4th spec.
At which point
A) You're adding a HEAP of classes which x primary stat x armor type dps specs, which was one of the reasons you cited against added a demon hunter as it's own class.
B) Risks stepping on the toes of other classes far more than a demon hunter will, at which point does the line between classes start to become blurred? Can they really make a melee hunter and ranged rogue spec fit, when already the rogue specs are in dire need of differentiating and the hunter specs aren't much better? You're avoiding adding ONE new class with three specs, that needs to avoid treading on the existing classes toes, to adding 11 talent trees that have to avoid stepping on our existing classes trees AND each others.
Adding another agility based CLASS doesn't matter much - there are still only 10-25 players in a raid (adding the demon hunter to a raid would be removing one of the other classes), adding new SPECS does matter, since those specs aren't taking up raid since if your warlock suddenly starts needing agility gear for his demon hunter spec, that's directly competing with the existing agility users in the raid - they've not gone anywhere.
You've also got all of the gear concerns I listed a few posts back, do your melee mages (as much as I'd love to see spell breakers), suddenly a complete mess to balance because they're meleeing while using spell hit, or do they need the melee hit levels, but require to get their expertise entirely through reforging? Is every class capable of playing ranged going to be pressured into that tree due to the overpowered nature of ranged classes on many fights? It's a tin of snakes that I doubt blizzard want to open.