8(TTIRAAS)

An Au where people from Satoru Mikami's world got soulmates and so does he but the only way to find them is by touch as once you and your soulmate touch(skin to skin is preferable but as long as it is a part of the body not covered by clothes it also works) a red line appears and ties you together before a ring appears in your finger(a silver ring with a small stone in the middle that somehow relates to your soulmate).

Satoru never met his soulmate and assumed he probably never will as his soulmate probably lived in another country, he had tried to date but it didn't feel right so he gave up on it.

He died without meeting his soulmate but when he reincarnated, he finally met his soulmate.

Who was no one else but Veldora(it could be Shizue but even in canon it seems like she was someone who could understand him and gave him the chance to look human again but not someone he would have loved like a lover) as the red line appeared as they touched each other the first time.

Veldora was confused about the line and the ring that appeared on him(Rimuru's ring was probably inside him as in slime form he has no fingers) but Rimuru, surprised that he met his soulmate in another world and said soulmate was a man, explained to him about his world having soulmates and how it worked.

Veldora was all for it, because he can be a bit innocent, and Rimuru followed along since they could first be friends before anything else.

During the time since Rimuru ate Veldora till he was able to free him, both got to meet each other better(Rimuru could communicate mentally with Veldora and visit him) and Rimuru finally came to terms that his soulmate was a man and that maybe he was bisexual and not heterosexual as he always believed.

How this changes the story? Well, Veldora would present himself as Rimuru's soulmate and Rimuru would put a foot down with Shion, Shuna, Chloe, etc as he had his soulmate and didn't plan to end with anyone else or get another spouse, when it comes to Rudra(Michael)'s plan it has two options: they succeed in controlling Veldora and Rimuru's fury is even worse then canon that he almost kills Velgrynd for taking his soulmate from him or they are not successful in taking control of Veldora as his link to his soulmate is much more powerful then the mind control thing.

I'm just imagining no one actually believing that Veldora means he and Rimuru are soulmates in the romantic sense but then get shocked at the end of all fights, they all receive an invitation for Veldora and Rimuru's wedding or are called to a meeting where Rimuru announces he and Veldora are finally getting married and he wants his subordinates help in preparing the wedding.