I mean, personally I think Warlock should be a prestige class (or set of prestige classes) for after you form a pact with some being of great power. Generally I think that's how a slew of character ideas that presume some sort of highly consequential event (like getting knighted or contracting lycanthropy) should be handled.
Warlocks are cool, and have lots of great design innovations, but the lore has not been a good fit in 5e. Having one class that just starts with a high ranking supernatural being as an npc ally is problematic. But also why are these supernatural entities all recruiting level 1 scrubs rather than trying to win over high level servants? Yes you could have a high level Wizard form a pact with Asmodius in pursuit of more magic, but his core spellcasting takes a nosedive, in favor of getting a better cantrip, which he has to cast with Charisma for some reason. Forming a pact for more magic should make you better at magic.
I'd prefer Warlock as a prestige class you can take after swearing service to a being of great power, that would continue your existing spell progression but replace your class and subclass features with Eldritch invocations and a pact boon.