Before I even ask this question, yes, I know that mechanically its because Blizzard isn’t allowing it.
However, for the most part, in lore, to become a Warlock all you really have to do to become a Warlock is be semi-competent with magic, and begin summoning and binding demons into service.
Dracthyr are highly competent with magic. Surely one could summon and bind a demon. So why can’t Dracthyr become Warlocks?
This is really the only reason why. 
They absolutely have the potential.
Perhaps one downside is Fel magic actually takes a toll on your body and ultimately makes you frail and weak. So that would essentially mean no flying, which could be enough of a deterrent.
Given it’s been stated that Dracthyr will have access to other classes in the future, it’s probable Warlock will be one of them.
As I understand it, one of the problems they have to overcome are the Dracthyr racials. They might also have to come up with some new casting (and weapons) animations for them too before they give the Dracthyr new classes.
Convert the problematic racials into Evoker class abilities. The only real issues would be the combat abilities. Or leave them in, but make them function differently for the race, with each class having a slightly different version.
Hmmm, IMO I think you keep your racials wings and tails for obvious visual reasons. Rest should be normal… both in Visage and Dracthyr form. I suspect only doing your racials would break the Visage Form. (IMO)
Heck I can even say that Dracthyr are candidates to be DH too! Devs just need to re-use the Void Meta of Sarkareth Raid Boss, give it a bit of Green Paint with Tattoo to match and BOOM Drakthyr DH Metamorphosis!