edit: I found this in my drafts haha, Its been almost 7 months since i wrote this Hello its been awhile since I last yapped here lol, I've been having fun on Twitter (well mostly because of Nelizx-San (我爱你哦朋友😭)
Recently I've been thinking about "what if Moon cradle was adapted into a movie/OVA."
Unfortunately the answer as we all know is closer to a no than a yes. It will never bear into fruition.
Ronye being the main focus of the novel is the first issue. Yes it serves for development of her character and subsequently the underworld, but Ronye is not a popular character and profits wise, a story centered around her will not bring much success.
And reading the novel, frankly speaking, it wasn't great. I say this while wholeheartedly while loving moon cradle (because of isscheta) I have reread it countless times because of them and own the physical novels. But honestly, The way Ronye cannot help but constantly yearn for Kirito throughout the entire side story was irritating to say the least. I can't say whether moon cradle even helped develop her character in the romance aspect. And I doubt most Kiriasu fans (and Kirito glazers cough) would like that very much either.
Besides the entire Ronye is yearning for Kirito every two pages especially in the first book, the Dark emperors arc is. Messy. To say the least. You would expect they helped aid in the world building of the human Empire, with them being the main villains, turn out to be a rather disappointing nothing burger? Yes it's not meant to be very long (only two novels worth of content) but surely they could've made more sense of a foundation for the Human empire and its tensions+politics? If how the world building went in direction the SAOUB moon cradle adaptation went, it's not great. I found that the motivations for the emperors were very lack luster and empty. Very filler like if I must say, which would bore the hell out of most SAO fans.
Lastly, there's Sylvie. An Original character from the game adaptation of Moon cradle. She is frankly speaking quite unlikable, and to add on she is just one of the billion SAO game OCs that almost no one cares about. But she has involvement in the emperors arc that makes her a more important character to the extended story
All in all I just feel like Moon cradle and its extension, the Dark emperor Arc, really doesn't contribute much to the Underworld. And with all these negative factors, I don't see a possibility of a movie nor an OVA. Yes there are some good moments (Tiese and Ronye earning their Integrity knights titles) (The Dark territory having some world building) (isscheta (of course)) Moon cradle by itself is not a complete story, hence it was extended with the Dark emperor arc with the game adaptation. The extension brings more content, which a movie cannot adapt in very limited time run. With all the popular characters having no screentime in Moon cradle, as they are all in the real world (other than kiriasu and perhaps Eydis) genuinely speaking, no one cares about the other characters. Unless they shift the focus less on Ronye and more on KiriAsu and their role as human delegates, perhaps I can see an adaptation. But will no longer have the essence of moon cradle, A story focusing on Ronye.