Well since in this universe you created sharingan was originally blue I think you should make sasuke purify the corruption in the sharingan bloodline and turn it into blue and if he is capable make sasuke trying to purify all of indras will's this way he can also allow them inside him and learn from them and get stronger I think sasuke could also take a part from world tree inject hiğh amount of nature energy and chakra into it and than eat or drink it to stimulate the transformation of his rinnengan of course he will not gain it immedieatly but it will trigger and situmulate the mutation and start the process of methamorphsis
