question what is the more or less conceptual niche of the witch?
So here are my brainstorm thoughts on this. They are a little raw...
Witches are spellcasters who learn their magic from some alternate way, and that alternate is a little... unsettling... to others. It could be magic from fiends, hags, deities, or the like, or some manner of folk "old" magic. Their magic is a little bit strange, even if the effects are like what a normal spellcaster produces.
As far as effects go, a witch's magic focuses on charms (robbing people of agency), curses (robbing them of ability) and polymorphs (robbing them of identity). There are secondary themes of nature (a witch is often rural and has some ability over the land), divination, and summoning (fey, fiends, shadows, the like). Tertiary, healing/herbalism and a touch of necromancy and general spellcasting (flight, etc.) round out the package. The thing I feel is that a witches magic is a little more subtle than traditional wizardly magic; some have said closer to a bard's list mixed with some of the spookier parts of the warlocks, but lacking the raw evocation damage that warlocks, wizards, and sorcerers are known for.
Part of the problem with warlock is most of the current warlock patrons that thematically fit tend to focus on damage (fiend is pure blaster, hexblade is about melee weapons) with only the archfey feeling like the subtle trickster in the mix (and that feels more Midsummer Night's Dream than Macbeth). Whereas wizard feels more at home with spellcasting, but none of the current subs really feel they nail the mix of witchy and instead focus on one aspect (divination, enchantment, transmutation) rather than give a general kit of witchy features.
There is a good argument to be made for a proper twenty level class to capture all the essence of the witch, but I think you could probably still fit it into a subclass or two (akin to how shadow mage is spread out among a few subclasses, or how divine/celestial magic is primarily clerical but also a sub-option for warlocks and sorcerers). I just don't feel the archetype is well served under the warlock's mechanics at the moment.