I love the idea of her being called Mavis. Or perhaps Doris!

My idea of M is an equally tough and capable, definitely not the weakling Jadis says she is. She is only 'weakling' because she is beaten.
M has powers and armies, and is able to use magic. Perhaps the only differences are that Jadis is extra tall and is prepared to use the Deplorable Word. It isn't clear whose kingdom Charn was before the battle, nor which of them was the elder sister - does this affect who got the throne? Jadis describes her sister 'at the head of her rebels' - as if Jadis were the actual Queen. But this doesn't preclude M from being the actual Queen against J's wishes, and J considering that she was the rightful Queen, and M only a rebel and leader of rebels.
M won't be as tall, nor as pale as Jadis, but she will be a light-skinned and dark-brown haired woman. Jadis by LWW has become very pale faced and very dark haired - which are part of the result of eating the apple in the walled garden.

While I don't personally hold to this theory, given that we only hear about "Mavis" from Jadis's retelling of the last days of Charn, it's possible that Mavis wasn't even a wicked person. For all we know, she was rebelling against Jadis because Jadis was a tyrant, and not just to take the throne for herself. I guess that's kind of an exaggeration of coracle's idea that it's possible that Mavis was the genuine heir and Jadis stubbornly refuses to admit that, even after Charn is destroyed.

Again, I don't actually hold to that theory. We know about the royal family descended into cruelty in recent generations. Jadis also basically associates cruelty with the ability to wield magic, and I don't really doubt from Jadis's story that Mavis did indeed use magic (who actually used it first depends on whether or not you trust her retelling). I just thought that'd be fun to throw out there.

I actually see Mavis as being very similar to what Jadis actually described: a jealous sister (older? younger?) who genuinely was willing to start a civil war - and employ the equivalent of a nuclear option - in order to get the throne. I agree with coracle that, while probably shorter than Jadis, she's just as capable as Jadis. I also kind of imagine that, despite the fact that she was leading the rebels, she walked into the palace in near-pristine robes and jewels - I get the idea that she wasn't the type to get her hands dirty when she had armies she could sacrifice instead.

I know how bad this is going to sound, but I can’t get it out of my head: Every time I think of Jadis’s sister, I think of the Queen of Underland. I just do. I know that there’s zero chance of that being C.S. Lewis canon, no more than fancy (unless a certain serpent survived the deplorable word and made its way to the wood between the worlds). But when I think of The Green Lady, as I like to call her, I see a different kind of evil than Jadis, but one that fits the family mold. Like Caspian is sort of a spiritual descendent of Shasta & Aravis, The Lady is like a spiritual sibling to Jadis. Maybe even one of the same crew...

While I don't personally hold to this theory, given that we only hear about "Mavis" from Jadis's retelling of the last days of Charn, it's possible that Mavis wasn't even a wicked person. For all we know, she was rebelling against Jadis because Jadis was a tyrant, and not just to take the throne for herself. I guess that's kind of an exaggeration of coracle's idea that it's possible that Mavis was the genuine heir and Jadis stubbornly refuses to admit that, even after Charn is destroyed.

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That's a REALLY interesting idea! Showing Mavis (who seems doomed to be called Mavis in this thread now) in a more positive light could be an effective way to really drive home what sort of person Jadis is, just by contrasting her and her sister.
Of course, it would be hard to portray her as heroic or virtuous and at the same time willing to violate a treaty to gain the upper hand, UNLESS it could be shown that she was somehow driven to it. (Then again, Jadis claims that Mavis drove her to use the Deplorable Word.)
I was just thinking what a cool scene it would be if there could be a sort of formal ceremony that begins the war. It could be in a dark, ancient, candle-lit hall, crowded with priests and dignitaries and generals from both sides, with the two Queen-sisters exchanging cold glares while the traditional oath that neither side will use magic is being read out loud.

It’s implied that the world of Charn was filled with very evil people. So I think Mavis should be very clearly evil. Nonetheless, I do think it’s safe to assume Jadis twisted the story to make herself sound more superior (even if in her own eyes) than she really was. Should there be a level of empathy from the audience for Mavis? Perhaps. But let’s not make her a martyr

As for casting, I really want Netflix too stick with an all British cast in general. Or at least as much as possible. Also, I’d prefer someone in their thirties-ish probably for both sisters. Rosamund Pike or Keira Knightly come to mind. For the sake of discussion, here’s a few more names: Hayley Atwell, Felicity Jones, Carey Mulligan, Michelle Dockery.