Hello! Yes, there is a theory that maybe Savanaclaw had been meant to be Book 6 (before the Diasomnia arc) and was moved at the last minute.

This topic was touched upon in a thread about Lilia back in January
https://twitter.com/YuuRei20/status/1610011750379958273

But the summarize:
The theory is trying to explain a few things that some people have felt to be “off” about the game:
1) Why Diasomnia’s cast had a significant role in only Book 2, with minimal roles in Books 3, 4, 5 and 6.

2) Why Lilia disappears during the climax of Book 2, saying, “stay strong until I return,” but never comes back.

3) Why Azul and Riddle were featured so prominently together in the prologue (Octavinelle would have become Book 2 and released simultaneously with the Prologue and Book 1).

4) Why Lilia had to be the person to drive Leona over the edge in Book 2.

5) Why Savanaclaw was originally listed in between Ignihyde and Diasomnia on the official website back in 2019.

6) Why there seem to be hints that Leona will have a significant role in Book 7 (this is another, separate theory unto itself).

Similar to how Riddle’s character was rewritten mid-production after everything had been approved ( https://twitter.com/YuuRei20/status/1626491706081439747 ) the theory goes that Yana got a better idea while the game was already being being made and things shifted about last-minute changing what was originally intended to be a struggle between two princes into Book 7’s current form (which is still in-progress and has yet to be truly revealed).

Book 2 being the original Book 6 would explain a lot about the curious things listed above, or maybe the Savanaclaw arc is actually, precisely where it had always been meant to be!

This theory might just be trying to fill in blanks left from various other technical issues that we will never know about.

The game was initially announced for a 2019 release, for example, but did not come out until March of 2020 (this pamphlet from August of 2019 promises the game ""this winter"").
https://twitter.com/twst_jp/status/1158302119876685824

And Yana has often referenced the struggles they faced during production.

Malleus and Sebek were not even in the game until over a month after it had gone live
https://twitter.com/twst_jp/status/1252493135629807616

While Lilia and Silver were not introduced until over two months later--
https://twitter.com/twst_jp/status/1264814046911459328
--so there was definitely some scrambling for the game to meet its 2020 release.

In the end, the story we have is Savanaclaw in Book 2 and that is that :>

But it is exciting to think that Idia and Leona—with their mirrored brother issues—might be teaming up together in Book 7!