Theory on Kayle and Morgana's Relationship and Origin
I assume that they will actually become part of the Shuriman crowd because Aatrox refers to Kayle as "unworthy of Ascension", the interesting thing is that on the map they are identified as Demacian.
I'm guessing they are born in Demacia, forced to flee because they are discovered as mages and run off to Shurima (the map lists Shuriman attitude towards magic as "covet"). Somehow Kayle gets roped into a bid to become ascended and this becomes her main goal. I don't want Riot to actually make her Ascended because that would require turning her into an animal god thing which they will not do for obvious reasons or create another subsection for the Ascended (baccai, darkin etc etc it's getting too crowded) to explain why she isn't an animal god thing, but their origin is something we will discover in due time and could honestly be anything. I give the people sitting firmly in the Mount Targon and Darkin camps due credit; they are also possibilities as this VGU is a thematic overhaul they are essentially going to be unrecognizable (RIP wings).
What's more important is getting the relationship between the sisters right. Like Nunus' VGU this should be one of the main focal points. It's important that we don't characterize Kayle as good and Morgana as evil. They should be more nuanced and play off each other with their own flaws and personalities. Now this may just be nothing but I noticed something interesting about the artwork in the Roadmap for Kayle and Morg
https://nexus.leagueoflegends.com/wp-content/uploads/2018/08/Roadmap_4-min_1u6ar6n4m193f7amowqg.jpg
Notice how the chain representing Morgana wraps around the sword (Kayle). What if this is allegory or symbolism for Morgana using her powers to suppress Kayles'? This would have been useful hiding away as mages in Demacia as one incident would have revealed them and led to imprisonment, exile (this might have been the case) or even execution. If this is true then this could inform us on the dynamic between the two.
I can see Kayle as being more impetuous of the two. Her lofty goals of becoming Ascended highlight that. She isn't going to have wings anymore obviously but the may still keep that as part of her design elsewhere (like a motif) as a callback, wings symbolize freedom. Freedom that she can now express having the yoke of Demacian oppression removed from above her head. Of course now a reminder of that cloud follows her everywhere she goes...
Morgana is the older sister and as such was always saddled with the responsibility of keeping headstrong Kayle in line (and her powers secret). To this end Morgana used her own unique magic to bind and suppress her little sisters nascent powers. She's much more reserved, cool and collected; she always had to be to stay one step ahead of the Demacian guards, an overly curious neighbor, even Kayle and Morganas' own parents had no idea. Of course now that they've been ousted from Demacia, Morgana still finds herself reigning in her sisters bluster even though she no longer needs to control her powers.
I like this set up because it allows Kayle and Morgana to have their relationship as sisters but preserves the pre-VGU tension between them in another form. Morgana was always Kayles' jailor as much as Demacian society was. She's a representation of the shackles that held her back her whole life, of course Morgana begs Kayle to give up on her insane Ascended dream and come away with her to Ionia or Freljord somewhere they can hide away together, but Kayle will not desist. Ascension is not just a means to power for Kayle (in truth she wouldn't really know what to do with all that power), but the caged bird always desires to spread her wings.
By the same token Morgana also has a bone to pick with Kayle, she is the one who got them discovered (of course it was Kayle). Kicked out of the only home they ever knew, disowned by their own parents, always on the move. Maybe their parents always favored Kayle in their youth, Kayle always was more personable and likeable while Morgana kept to herself, the burden of keeping them safe never allowed her to be herself fully. This has culminated in a resentment of Kayle starting to brew in Morganas' heart.
Aatroxs' VGU as much as it as reamed for being "edgy" by the player base actually stands out to me as one of the best for me. It uses Aatroxs' character who is unrelatable on it's own (I have no reference point to know what's like to be a transcendental being trapped in cutlery) is presented in a way that is very relatable. What's it's like to be depressed, to feel trapped, self-loathing and suicidal (my own brother is bi-polar). He's one of the only character in League that not just exists as a character but has some kind of statement to be made about the human condition.
Kayle and Morgana could have a story arc that also speaks to me: Growing apart. They undoubtedly still love one another. They are all they have left in the world, but the old happy memories of playing together as children have given way to a more complicated world where they find each other, strangely, now at odds, even though they both need each other.
What do you think Riot will do with the new Kayle and Morgana? This is just my personal head canon, as I never found Kayle and Morgana as immortal enemies compelling or relatable. Are they Targonites, celestials from the silver cities or Darkin? Let me know your thoughts.