We all have classes we want to see beyond the core 12, and I'm sure we're all wondering about how those classes might be in 2nd Edition. The Witch isn't the only one of mine, but it's the one I think I can most easily approximate for the playtest without multiclassing or having to, you know, fully get into game design. These are my thoughts on how to go about it, based on what we know to date. This could be made laughable once we get the full playtest. I expect that it will, if it isn't already, but I don't care.
(Note: I don't intend to use this for running the playtest. That would ruin the point. It's solely for fun)
Thematically, the witch is a bit of an odd duck compared to its arcane kin. The magic doesn't come from innate power or general study or both. It is facilitated by a patron, which will doubtlessly have its own reasons for doing this. There is a general witch spell list, but the patron also teaches a particular set of spells. Also, the patron doesn't directly interact with the witch. It sends a familiar to serve as intermediary. The witch must commune with the familiar to prepare spells.
Let's get down to the mechanics. The obvious choice is to use the wizard as a base chassis. Use the proficiencies, saves, and so forth for the witch. However, instead of arcane focus, you get a familiar. I'm doing away with arcane focus because a spellbook shouldn't be a good trade for a feat, but a familiar could be. And yes, just as a wizard can spend a feat to get a familiar, a witch can spend a feat to get arcane focus. They'd use their familiar instead of a bonded item.
Wizards have schools, and witches have patrons. I would use sorcerer bloodlines to approximate the patron. The bloodline spells would serve as the specialist spells, and the bloodline powers would serve as the hexes. This has the advantage of being straightforward, and the downside of being constrained. You get an extra spell slot per spell level, but you have to prepare that particular spell within it. Specialist wizards have several options per spell level. A witch wouldn't, but that would be the tradeoff.
There could be a workaround, though, if you're willing to spend the feats. Clerics can take extra domains, so let's give the witch a similar option. Call it "Extra Secrets," or something like that. It gives them access to the spell list and basic power of another bloodline, with additional powers being unlocked with feat investment. You could even have them take Expanded Patronage a second time, much like clerics can have up to three domains. The fluff explanation would be that the patron is giving the witch access to extra secrets. Patrons are less defined than gods or bloodlines anyway.
What spell list should the witch have? Much like the bard, the witch has access to spells not found on other arcane lists. We could do what the sorcerer does and tie spell list to bloodline/patron. It should correspond to the initial bloodline/patron, so as not to get too broad. All patron spells would be prepared in normal slots as well, so taking Extra Secrets would provide additional options as well as making no too witches alike.
Finally, the patrons should have anathema. They don't have to tutor the witches, after all. If the witches anger them, the familiar should go away. The witch would keep all prepared spells and remaining spell points, but wouldn't be able to regain them until they make things right. If that means a witch gets down to just cantrips, oh well. "Don't turn down a chance to make money or increase fame" would be thematic for a draconic patron, and "don't turn down a challenge" could work for an imperial patron.
Anyway, those are my thoughts based on what we know. They're likely to change once we have the playtest (and I'll be shocked if they don't). What do you think?
Thematicly, I could even see a herbalist, potion brewing witch as an Alchemist. Maybe with a magical archetype or multiclassing to get some cantrips to use as hexes.
Necromancy Wizard, Familiar for first feat, and focus on the curses and debuffs from that. Arcane necromancy doesn't get the healing side of things, and the playtest doesn't include creating undead minions (that's reserved for the final book), so you should be left with a lot of very nasty stuff to emulate that side of Witch. You're still an Int-based full caster with a familiar who curses people.
Yeah, I don't think it will be hard to convert a PF1 with to a PF2 wizard as is. The wizard is becoming similar to the witch in a lot of ways. Less spell slots, more at will abilities, more modular powers selected every other level, more potent familiars who can store spells, fly spell now emulating the flight hex... Provided we have a debuff cantrip similar to Evil Eye I think they will be in good shape. Take something to get nature as a signature skill probably.
It seems the OP was more focused on a hypothetical future class or creating some kind of home brew witch, but I feel it worth pointing out that you'll be able to get the same mechanical feel of the witch with the wizard.