Rashta
Acnologia Fairy Tail, Grimmjow Bleach, Asakura Hao Shaman King , Esdeath Akame ga kill, Takasugi Shinsuke Gintama, I don't know how I would put it into Acnology, but I think the others would be easier to unite, Grimmjow would have to be human in this world and everyone can create a power like the ones they originally had with the techniques and souls of beasts.
Since you went Gyutaro from demon slayer would be cool to see Sesshomaru from Inuyasha as one of his guards and unlike Gyutaro, he would be his right hand man and if you didn’t already ofc we need a female to be a member of the four stars we can do a fan favorite Esdeath or an unorthodox choice from another Chinese novel soul continent Qian Renxue very smart, cunning, and if she falls in love loyal but she is very arrogant.
maple from bofuri would make for a great shield person, douma idk I just like him, zeref he can be a death cultivator maby kaguya from naruto I love her character but she is never used in fanfics, zhonglie from genshin he would be a great butler. these are just off the top of my head and characters I really never see have a big role in fanfics