And, while I have a warlock languishing at level 85, I'm jealous. Why should warlocks have all the class quest fun? I definitely don't think, though, that these warlock quests should be opened up to other players, just in the interest of completeness. The great thing about these quests is that they're tailored around warlocks and their class abilities, other classes would simply dilute that experience.
No, I think this should herald the start of things to come. The start of epic questlines appearing for every class, old and new. But, importantly, they have to offer something similar to the warlock quests. I would want them to be similarly difficult, similarly demanding of competence, and similarly require players to reach back to the darkest recesses of their spellbook. However, a quest of that magnitude cannot offer something required for raiding, for PvP, for character advancement. It has to offer something like green fire, desirable but not necessary.
And that's where the difficulty comes in. What could these hypothetical class quests offer?