Yaarel
🇮🇱 🇺🇦 He-Mage
If it was just power source (like arcane pacts, or divine blessings, or primal spirit mediating, or elemental channelling) I still think it would be fine to consider it a single class.
I especially like the option of choosing the power source of any class. Not only are the styles of witch so different from each other, the theories of magic can be so different from each other. Here choosing a power source can help pick one that is correct for a concept or close enough.
For example, to do a Norse Witch, meaning a Volva or Seidfolk, simply granting the Psionic tag to a Bard, is remarkably accurate. Any specific information if necessary is just a new spell or a new background. Seidr is strictly telepathic mindmagic, but the people who do it, also tend to master other shamanic traditions as well, including healing and the sight, outofbody and even shapeshifting. The psionic disciplines of Telepathy, Prescience, and "Psychometabolism" which I call Shapeshift, are central to the Norse Psionic, but Telekinesis is rare.
And to do a Celtic Bard, who is similar to Volva, the Bard class is already remarkably accurate. For the Bard, I probably keep the Arcane power source in the sense that I get a strong secular vibe from the Bard, or possibly go Primal in the sense of being infused by the magic of nature. I view the Bard as Prescience (including blessing by praise and cursing by satire), Telepathy mindmagic, Shapeshift, ... and potions.
And so on. If it is possible to officially swap a power source, it goes along way to get a character concept exact.
Maybe like Priest is a Background, Witch could be a Background. But really because witches can be so different from each other, I doubt there could be a single Background to meaningfully unify all the concepts.
But Witches are also different in their approach to the nature of magic.
I can think of two classes that scream Witch especially in how they reach out to different power sources:
Warlock just needs to add a few more pacts like The Primal Spirit, The Divine Pairing, The Elements.
Sorcerer is almost good as it is, but could use an explicitly Fey origin, and perhaps a Cosmic Magic source (as well as Frost Sorcery, Flame Sorcery, and Stone Sorcery for more elemental themes beyond Storm Sorcery).
I view "Primal" as animism, thus a kind of Psionic that is more attentive to the minds of landscapes, plants, and animals, but also the minds of skyey phenomena, including sun and moon and weather patterns. Generally, Psionic tag is fine, but specifying Primal can be helpful, especially for flavor. A Primal Warlock might be Primal or Psionic, and the pact itself might be some kind of personal magical transformation that occurred when the shamanic sensitivities first awoke.
Divine Pairing? Wicca? Divine power source?
I am less satisfied with the 5e Sorcerer. In the previous editions it was the Nonvancian mage, and in 4e and 5e it has an identity crisis. I would prefer if we had the Psion instead of the Sorcerer. Nevertheless the Sorcerer can emphasize its "bloodline" trope, and be a design space to explore Nonhuman ancestors. I havent used the Chaos archetype since it is lessso an ancestry, but maybe it could be some kind of Elemental ancestry. Various bloodlines are relevant to various witch concepts.
I tend to be happy with how the Druid class handles Elemental tropes. I use the Druid to represent the more elemental aspects of nature, like Frost Thurs and Wind Jotnar. I also use the Druid with some accuracy for various Alchemist traditions. I view the reallife Druid as a priestly caste, having a worldview that is simultaneously polytheistic and protoscientific, perhaps with no distinction between the two. If I remember correctly, a report says they can take a priestly vow to not fight by weapons but to fight by magic. Generally, Celtic magic is dreamlike experiences with vivid symbolic imagery. I suspect all the traditions of complex and convoluted potion ingredients comes from the Celts and perhaps specifically from Druid sacred traditions.
If a Warlock makes an Elemental pact, I am unsure what that might look like. But it would be easy and balanced to modify the Eldritch Blast to whatever damage type is appropriate to the Elemental concept.
But again, there's Artificers that feel very much in line with historical Euro-American Witches as alchemists and healers that were demonized by a rising male industry of medicine; there's Bards who lean into the ecstatic and performative side of witchcraft and healing, while also being dabblers and seers - there's a reason that Troubadours, Nuns, Witches, & Concubines share a history of countercurrent creation in medieval Eurasia. And there's Druids and Wizards who have each inherited much of the tropes of witchcraft we have in popular fiction today. And of course there's always Clerics and Monks; the lines between witch and faith practitioner is often a line drawn by oppressors. Or, the rising dominant religious communities recasts magical persons as religious ones; Goddesses become Saints or Angels, Witches become Nuns or Priestesses reinforcing the divine right of the heroes they interacted with.
I enjoy your comments about the history of various witch traditions. I know various indigenous cultures getting demonized as "witches" has inflicted much suffering. But I tend to not focus on the tribulations. I am more excited about how amazing these ancient cultures are, and hope they continue to survive. It is impossible to avoid evolving, but I hope each culture can maintain authentic continuity with ones unique heritage.
Look at Morgan le Fay from the Arthurian cycles. She's been cast as a witch, a sorceress, a depraved nun, a misunderstood priestess of an old religion, a misunderstood nun or priestess of the current religion, a goddess taking human form, a demigod, a fairy queen, or a female magus of some kind with powers similar to Merlin (himself a character that has been expressed in a multitude of ways and could be represented by the D&D Wizard, Bard, Sorcerer, Warlock, Cleric, Artificer, or even Fighter or Paladin or Ranger if we're talking Emrys or Ambrosius).
When it comes to Merlin and Morgan, I view them as a blend of reallife persons and concepts. I focus on how the narratives were moreorless plausible to the writers and the audiences who read about them, thus describe reallife worldviews of the period.
Regarding the archetypes of Merlin and Morgan. I view Merlin as a Bard, and it is almost that simple. A Bard can also be a political advisor and even, potentially, a military leader.
Regarding Morgan, "le Fay" says it all. I view her as a spirit of fate, the kind that personifies magic itself, and specifically what came to be identified as an "Elf". Albeit, she is the British version of Elf that synthesizes the Celtic Sidhe land spirit and the Frankish Faie personifying fate and magic.
Merlin himself is probably a Half-Elf, where his "incubus" father is almost certainly the shamanic Elf that relates to the Scot witch tradition.
I view both Morgan and the father of Merlin as animistic land spirits.
Magic in stories don't necessarily comply with each other. Maybe it's an argument for combining magical classes and calling it all just Mage with different flavours, but I think 5e resolves this best of any D&D edition: it's able to provide us with tools to enact classic character archetypes and then to finetooth it with subclasses. The answer thus would be to give us 5 different Witches that all have different flavours of Witch and sit within different classes, or to find the witchiest of witch ideas and make it a Strixhaven-style subclass that thus can combine with various classes to form the best idea of what a witch is to a given player. But doing so would also require guidance to figuring out the systems mastery, and I recognise that some people just want to be a witch and be done with it (much like some want to be a champion fighter and be done with options paralysis). In that case, I would want chapter 1 or 2 guidance directing them to classic archetypal characters, and witch could be one of them, saying if you want to play a witch of that classical variety, choose the Warlock class, etc.
Using the classes to handle the thematic mechanics, but then allowing one swap the power source, helps to approximate certain concepts.
A power source is a tag. Mostly, the tag has no mechanics in and of itself, but other mechanics might refer to it. However, for me, Psionic spellcasting is innate, and always lacks spell components.
A Background might help. I use one for a Norse warrior mage, to make sure certain spells known are available (generally equivalent to the Abjuration school) are available to a spellcaster class.
Strixhaven opens up a new design space, where certain witch concepts might work as an archetype that several classes can share.