Slightly related to this is what to do when the warlock acts against the interests of the patron. The patron doesn't even need to be evil, I just feel like the standard lore could stand to include some guidance on what happens when the relationship between a caster and the source of his magic goes sour. Same goes for clerics.
Yeah.
Demons and Devils should be obvious. Faust had to pay up. And why wouldne an evil Demon/Devil just leave a Warlock High and Dry jut for entertainment's sake?
Fey can be very fickle, and even the Good aligned Patrons wouldn't be above making the Warlock quest or demand compensation over some mythical slight. What about some powerful fey that is unimaginably beautiful, took notice of the Warlocks affections, granted him powers in exchange for passion and devotion? Starts out all fun and games but what happens when said Warlock is near death on a campaign, needs that long rest, and the Patron demands an evening of passion? If he refuses, does he lose his powers for insulting the object of his devotion? For breaking his promise?
Hexblade could be trickier, but could throwback to AD&D with super-intelligent magic weapons that are pretty representative of their alignment. The weapons usually end up possessing the user.
GOO should be more insidious and warrant more fear and regret. It seems the Patron with the most latitude for theme because they seem so impersonal and distant. But what happens when they arent?
Warlock thinks he tapped into something that doesnt notice him siphoning a little power and all seems well. Later finds out that whatever he tapped into noticed and may have planted those ideas in his head in the first place. Then i demands he start a cult, where he finds some forgotten hamlet, take all the townsfolk, cut out their tongues and insert a parasitic but in th tongues place. He does this to 50 people and then gets to go back to whatever he was doing. He comes back 2 years later and all the townsfolk are otherwise normal and healthy, but built this big metal dish the size of a castle and it points to a very ominous star. Good aligned characters should have serious doubts about what they did.
But no, a lot of untapped theme and no mechanical structure for when things go bad.