One thing you can do with warlocks is to take the idea of pacts and bargains and run with it. In this view, there's always a cost for warlock spells. When a fiend grants you power, he isn't granting you his personal power--he's giving you the password to his soul account, so to speak, and every time a Fiend Pact warlock casts a spell, there's a soul somewhere who screams in agony as the soul grinders squeeze a little bit more juice out of him to power the spell. When someone makes a pact with Cthulhu or the Unseeing Eye (with or without Cthulhu's knowledge), the forbidden lore they're accessing is forbidden because it chips away at the reality of the world, bringing the world's dissolution (back?) into chaos that much closer every time you use it. When someone makes a pact with Mab the Queen of Air and Darkness, their spells, like Mab's own magic, are consuming the vitality of the natural world, and Mab will insist that the power be used in such a way as to create a net profit for her, by removing threats to her power source.Xvarts have been a lot of things over the years.
They started out as some kind of being that was half-way between a Kobold and a Goblin (both of which have gone their separate ways). Later they were repurposed into Gnomes who were experimented on by Fomorians, and made a pact with some Hags to escape.
Anyway, the most interesting thing about Xvarts is their master Raxivort, who is explicitly a demigod instead of a fiend (at least now), but still grants warlock pacts to his direct offspring, who worship him, instead of making them clerics or something like that. In contrast, Graz'zt is supposedly still more powerful than him, yet isn't a god of any kind yet?
This is the kind of thing that makes me loathe the design lore for Warlocks in 5e. They overlap too greatly with clerics, which doesn't make sense. Why would a divine being make a pact when they can have worshipers, what is the gain for them at that point?
Raxivort doesn't have personal power to loan to his warlocks, but the artifact he stole has a credit line built in, which doesn't seem to have a limit--he keeps pulling power out, and so do his warlocks, and the debt keeps growing, and he worries sometimes about where it comes from and what it might cost someday to settle that debt, but so far no one has ever asked him to pay so he just keeps drawing upon the power because he wants treasure and a good life today...
From this perspective, wizards take power from natural laws, clerics are given power by those who have it, sorcerers embody power within themselves, and warlocks (and their patrons) borrow or pay for power like nobles borrowing gold from rich merchant princes at high interest rates.