Bear with me. The first few paragraphs are exposition and then I'll explain what could happen if the show did the Morrígan.
The Morrígan is a goddess from Irish mythology. She's very unique in the sense that she is usually represented as three identical sisters. She's also been linked by many Arthurian scholars to Morgan Le Fay (or Morgana La Fay... whatever you wanna call her lol). She's a goddess of war and death, associated with cows, crows and dogs. You can pretty much learn all you need to about her powers by watching this music video.
Another entity represented as a trio of sisters is... Well, the Fates of Greek Mythology. You can see what Disney's interpretation of the Fates looks like here. Contrary to popular belief, the Fates did not override free will. They just killed you when your time was up.
Now Brave and Frozen in the main show and Star Wars in Wonderland have shown that no Disney property is off-limits. Unfortunately. And we all know three lovely sistahs that Disney fans love oh, so much.
With the Morrígan being a link to Arthurian legend and Camelot, as well as the Fates being a link to Greek Mythology, it would be pretty awesome if the Morrígan was introduced into the show as an alternate name for the Fates, who they themselves would be introduced as individuals, sharing the names of the Sanderson Sisters.
So maybe the Fates could be introduced as the all-powerful Morrígan, a coven of primordial sister witches as old as the first story who control fate and destiny as one entity, while presiding over their own domain. Each one could preside over an abstract concept that would line up with the three magical laws: Sarah, the goddess of love. Mary, the goddess of time. And Winifred, goddess of Death. With Hades serving as the King of the Underworld instead of the God of Death (which I don't believe he technically is in myth, either. He's the God of the Underworld, not of Death itself.)