My answer, and one that occurred to me because of comments by @[email protected] in this thread is the warlock.
In my view, the key aspects of the warlock are:
- It must have a patron which controls its access to magic
- Its magic must be of a sort that, to an outsider, could easily be confused for a wizard or sorcerer
- Nevertheless, the mechanics of its magic must feel very different to play from a wizard or sorcerer
D&D 5th edition does this well with its spell slots being short rest based and always at maximum level, but far more limited in number than typical slot casters. It casts many of the same spells as a wizard rather than having an entirely different system like Pathfinder's Kineticist or (presumably) runesmith, but by preparing and casting in completely different degrees to the wizard. Whether Pathfinder did it "slotless".
The Witch is probably the "best" option for a warlock-like experience so far, and the description of the witch as having a patron is probably the biggest reason I think we'll never actually get a warlock. But the witch does a very poor job of feeling like a warlock. I don't want a pet, or to cast spells through a familiar. The actual spell progression is too vanilla. And way too many of the feats are too explicitly "witchy", like cackle, cauldron, living hair, and eldritch nails.