Given their penchant for casting Lost actors as villains, I’ll nominate Evangeline Lilly in her place.
She was the one actress my boyfriend and I could not stand from Lost. Watching those Kate backstory episodes was not fun for us. It was painful. Besides which, Morgana comes from Arthurian Legend and that’s British lore. If you’re going to cast an actress to play someone British, mine as well cast a British actress. American actors are not known for doing good British accents, but it’s oddly easier for British actors to do American. Not sure why.
Lancelot is a French addition to the British legend and he was played on OUAT by the American Sinqua Walls. Conversely, The Wicked Witch of the West comes from an American novel and was played by an English actress. Or the Scottish Rumplestitskin (German) and Australian Belle (French). Clearly national fidelity is out the window already.
Keeper of Zelena's witch hat and the Black Fairy's wand.