I think that's a very wonderful project of yours :DD I wish you the best of luck in making happen! It does sound very ambitious though, considering the material you'll be working with

Keeping track of everything Anzu and Ten-kun related in stories is already heard enough as is (even I'm nowhere near complete in terms of knowledgeable on everything about anzu across all the stories) and I'm curious about what story you'll give it. While I'm both interested and excited to know what you'll do with that, I just hope you aren't biting more than what you can chew you know o(╥﹏╥)o

Theres also things you might need to note in tackling their story, like how most of Anzu and Ten-kun's familial situation is still unclear besides hints we've gotten from the characters (close as kids, complicated family situation, he was distant in ES! but cares for his sister and is curious about her in EG! etc.).

There's also a really cool timeline interpretation by @ATMforTKS that I myself have started to internalize when I read stuff from EG and ES that comes from staff comments about the two games being in different world lines, meaning that events in either games shouldn't be automatically considered as a canon event in the same world. Although personally while I can acknowledge that both games are in different world lines, I do still consider some events as occurring in both worlds, especially most things involving anzu in the present day since information from EG doesn't contradict itself in a way hurts reading the text too much, but it does account for anything their past that one might raise an eyebrow on (things like the differences in the two game's setting and characters, anzu's incredible feats during her stay in Kimisaki, circumstances about Ten-kun's etc). Of course you're very much free to take both games as is, they don't really hurt each other continuity wise too much unless you're very particular or nitpicky about those details

Plus, as much as I try to hype up Ten-kun because of my admiration of him obtaining his own agency and becoming his own person independent of the player by the end of his game, it still doesn't change the fact that he's still much more of a blank slate than anzu is. it sort of helps that both siblings are described as similar to each other in terms of behavior, but there's still a lot more information on anzu that can be used to help write her character in a fun yet canon-compliant way, while much more information would have to be interpreted for ten-kun. but also i'm not gonna pretend i read a lot of engirls so it's possible i missed his characterization while reading stuff (·•᷄‎𐃷•᷅ )

I also don't know (or in this case, curious to know what you'll do) how their stories can be told without straight up just speeding through their respective stories hahaha too many things happen in different times that i can imagine you'll have to combine different events like what the anime had to do. and making your project a musical sounds super tough! i don't know know a lot about making music but i do know it's a lot of work and creativity so you have all my respect if you could pull it off

i think it's admirable that you want to tackle a rewrite of the 4piece arc, and i hope you have fun with that :)) i'm gonna be honest and say that i'm very behind in reading the stories of that arc so i don't want to comment on how much it does for anzu's character or what can be changed in the story. don't know if you mean you'll combine that rewrite with your current project or if it'll be a separate one (since both seems like it'll have a completely different cast i think) so i just hope you'll keep your plans realistic and manageable

anyways, thank you for sharing your project with me! I wish i could volunteer to help but my life is pretty packed as is, but i do enjoy exchanging ideas and can help promote it if ever ദ്ദി(˵ •̀ ᴗ - ˵)