Imagine a Ritual that allowed ANY spell to be learned...
The default Spell Sharing Ritual would be Uncommon,and distinct for each class learning the spell as well as each spell.
The availability would be up to the GM, meaning anything could be possible, but nothing is given.
Learning a spell via the ritual would not automatically teach you the ritual itself.
Now we look at the Witch.
Witches get the Spell Sharing Ritual for any spell they know
These Rituals are how they learn each spell from their Patron.
Spells known would be gained at the same rate as a Bard.
Additional spells known could be acquired by feeding Scrolls or Wands to Familiars.
Each spell could be chosen from any spell tradition.
Coven members perform the Spell Sharing Ritual among themselves for free, this encourages Coven membership.
Patrons can be nebulous or specific,known to the witch or not.
Official Witch Patrons list what Hex and Spells are available,and by what Tradition the witch can cast.
Witch players can choose their casting Tradition, Spells and Hexes, thereby shaping their own unique Patrons.
Witch players could cherry pick the most effective spells, or they could choose thematically.
Either choice had it's own reward or punishment.
Seeking out additional spells known that can then be shared with or hidden from the Patron offers role-playing opertunities.
One Hex would be gained at 1st level.
Other Hexes would be added via feats, in much the same way as Bards gain Compositions.
Hexes would be mixture of Cantrips Hexes and Focus Hexes.
Hmm. Replace magic traditions with ritual learning. That makes sense for communal witches but hurts the self taught variety. I don't see Granny Weatherwax being limited by what she is taught. Adding to known spells via eating scrolls just means more expensive spell acquisition. So this means broader choice than most at the cost of fewer spells and less loot.
I like this flavor for some witches, but this will be painful in PFS and published modules. As you are off saving the world, there will be little/no interaction with your coven, so you will feel the downside of the limited choice of spells. I think this will be mostly for NPCs, or home games.