Mechanically, I'd like a 3e warlock. I like the idea of a magic-user with just a pile of powers they can use all the time, for any reason, in any context, all day long. Only at-wills! Not a "striker" (a very narmed choice).
I'd like to steal the curse boons from 4e, though, and the various pacts. IMO, the idea of a warlock as the "mage granted magic from an outside source" is great, and maybe should even overlap/pervert cleric mechancis in a few ways. Warlocks are great places to locate powerful summoning, charms, and illusions, too.
I'd personally make warlocks really solid at the "social" pillar: suave, dark, sophisticated, persuasive, with magic to manipulate those they interact with.
That's kind of a bigger change than maybe first-grok 5e can handle, but I'm not interested in a middling striker that works mostly just like every other character but with magic pew pew lazers/knives/whatever. I do like the idea of basically the warlock as an "anti-cleric," forming pacts with supernatural beings, and twisting the trust and gentility of the clerics into a manipulative, selfish kind of persuasion.