I love the class, and it's mechanics.
However... It is also one of the classes that suffer a bit from lack of prestige classes.
Enter, the Hellfire Warlock.
An awesome PrC for the good old lock, with few drawbacks.
That is, however... till you get to the Lore.
- Nwn2 wikia.The hellfire warlocks are a secretive group of specialist warlocks who have mastered hellfire, a dangerous energy found only in the Nine Hells. By tapping into this infernal power, these characters learn to infuse their eldritch blasts and magic items that they wield with the dark power of hellfire.
- Fiendish Codex II.Hellfire Warlocks in the World
"The offering of hellfire is yet one more way the fiends have worked their claws into our world. Each mortal who thinks to master this fell power is yet one more fool offering his soul to the Lord of the Eighth."
-- Alód the Traveler, planeshifter
Now, if you know anything of Mephistopheles... you know that he is the creator of Hellfire, and... guards it's secrets fiercely.
"...To make matters worse, he has mastered the essence of Hell, channeling it into a foul destructive energy called hellfire. With such tools at his disposal, it seems Mephistopheles will one day make good on his promise to rule in Asmodeus’s stead. Thankfully, Mephistopheles does not exert the same influence on the Prime Material Plane. Many mortals confuse him and Asmodeus, believing they are one in the same. This frustrates Mephistopheles to no end; above all, he wants to be worshiped as a god. Still, he has a number of small cults that revere him as the god of hellfire."
What I am saying is... And not to make life more difficult to warlocks... but;...Recently the Lord of the Eighth has been devoting most of his time and incredible amounts of energy to his pet project, that of mastering and experimenting with Hellfire, a new form of magical fire with new and interesting properties. This obsession with the enterprise has caused a virtual upending of Cania's power structure, with the Ice Devils that once stood at its peak forced to migrate to the colder mountains on the fringes of Cania.
Mephistopheles' goal is the expansion of his material plane cult by offering mastery of Hellfire as an incentive. However, the expansion is taking a toll on his soul-harvesting efforts, and he is being forced to borrow divine energy from the other archdukes, particularly Dispater and Levistus at an alarming rate. Mephistopheles is risking much in his gamble on Hellfire, and if it pays off, his mortal cult will be larger and more popular than any other archduke's, including Asmodeus'"
Would it not make sense if Hellfire warlock required you to worship Mephistopheles as a deity?
(and yes I am aware this would require mephy to be introduced as a choice)

(Not to mention when you keep meeting other characters having the super-secret-hard-to-master-magic-of-hell-itself >.>)