This is based on something I am working on but I am not sure if I will actually post it instead of letting it reside between all my other wips.

So, we know that Ciel looks like Rimuru but with red/pink eyes and about the existence of Emil, right? So, what if the three of them were Milim's younger siblings and triplets?

And no, I don't mean that they were born before Veldanava and Lucia died but that at the end the three of them were made by the magicules/souls/whatever that belonged to both Veldanava and Lucia joining together and creating them.

But for some reason both Emil and Rimuru reincarnated in different worlds while Ciel was kept safe by the VOTW(voice of the world).

When Satoru Mikami was reborn in Tensura, so were the other two with Ciel as Great Sage and Emil...I actually am still not sure how Emil would be seeing he technically was born from Shinsha's wish but maybe he would be reborn in the future instead of at the same time as Rimuru and Ciel(or maybe Emil could be Beelzebub/Void God Azathoth if it also became a manas?)

But anyways, I'm still working on things but after the last fight the VOTW would announce that Ciel, Rimuru and Emil were ready to begin their final evolution to become Veldanava's heirs(because I like of them being triplets and doing most things together which also includes becoming gods together but each would have a specific things to take care of).

And while in most fics/reacts/etc people would just go along with it because Rimuru is involved, it leads to them trying to find why those three were the chosen successors(and why just appear now instead of being the ones overseeing the balance in Guy's place-yes, this is the biggest reason why Guy joined this group xD) and somehow they find out that the three of them are technically Milim's little siblings.