Yet another idea for a TTEOTM fanfic I'll never come around to write.
Cang Jiumin is a cultivator, the adopted son of Master Zhaoyou. His world has been on literal fire because of the Phoenix Demoness Li Susu, daughter of the Ancient Devil God's Lieutenant Di Mian who managed to snatch the Evil Bone along with the Sky-Slashing Sword when the Devil God fell. Even if Di Mian was trapped in the Barren Abyss, he managed to free himself and found his daughter's egg, powering her with the Evil Bone's power he corrupted Li Susu's original pure nature, turning her into the Phoenix demoness who brought destruction upon the world ever since she hatched.
When Li Susu decides to attack the Immortal Sects to obtain their artifacts, she and Cang Jiumin clash for the first time, and she seems to recognize him somehow, much to his confusion. She attacks him anyway, but he's protected by Ming Ye's dragon scale and able to access to the Mirror of the Past's power.
This leads the elder Immortals to believe he must have some godly ancestry or protection and can use the relic. As such, Zhaoyou entrusts Cang Jiumin with the task of going back in time to make sure Li Susu won't be corrupted by demonic power.
Now, he finds himself in the body of Tantai Jin, Ye Xiwu's husband, and here I have two routes:
The past background is the same: Tantai Jin is the hostage prince of Jing who managed to marry the favourite daughter of the Pillar General of Sheng, Ye Xiwu is vicious and shows every possible bad trait that will lead to her easy demonization, so he has to try to awaken her best feelings so she can learn to love the world and respect others.
Tantai Jin is the favourite prince of Jing, even though he was born the son of a concubine. When he brought an end to the long-time war against Sheng thanks to his wit, he was ordered to marry Ye Xiwu, daughter of the Pillar General of Sheng, to consolidate his power in the foreign land where he's staying as regent (basically, his father let him have Sheng to compensate the fact he can't get the throne of Jing due to existence of his older, legitimate brothers). Obviously, Ye Xiwu hates him, is spoiled, and all the drill.
which is actually 2.1: Tantai Jin is the favourite prince of Jing who is friend with Xiao Lin and his marriage to Ye Xiwu was wanted to ensure a more lasting peace (like in his dream in ep 4). Ye Xiwu, who thinks Tantai Jin will never get the throne due to the existence of his older and legitimate brothers other than the fact he has a reputation of being weak and meek, despises him and the marriage.
Plot 1 would retrace some steps from canon, with Cang Jiumin having to try to elicit pity and affection from the tyrannical Ye Xiwu from his disadvantaged place as a hostage prince and unwanted husband. The moment he transmigrates is when he dies kneeling on the frozen lake, he's "saved" by the benevolence of Grandmother Ye.
Plot 2 would require some more imagination and delve into palace matters and politics (which are not my expertise) but would make a more novel plot.
In any case, they should come around to discover a few things about each other:
Ye Xiwu's life was not as nice as it looked, despite being the only legitimate daughter and most spoiled, in fact, she was always overshadowed by Ye Bingchang ever since she stole Pian Ran's Love threads. Ye Xiwu genuinely loved Xiao Lin and had her heart broken by his indifference, other than the fact Ye Bingchang used her newfound influence to push her sister out of society.
Tantai Jin is a tribulation of Cang Jiumin (like YXW was LSS), but he's not the Devil Fetus here. Instead, he's the lost son of Ming Ye and Sang Jiu, who she lost without even knowing she was pregnant when she saved Ming Ye the last time in the Ruo River (hence why he was born from Concubine Ruo)
Ming Ye drags them into the Bo're Life to show YXW who her father really is like (Di Mian did kill Chu Huang after all, which he lied to LSS about), and also the risk of allowing herself to demonize like Sang Jiu did. For his son, he wanted him to know the real story and see his mother, but also apologize and warn him not to make his same mistakes of being too cold and distant with the woman he loves, along with the hint to the presence of the Evil Bone in LSS and the need to replace it with an Immortal Marrow.
After their deaths (which will depend on how the plot develops) Cang Jiumin comes back to his life in Xiaoyao sect and is surprised to find that Li Susu is now a new disciple of Qu Xuanzi, leader of Hengyang sect.
This could be the end... or not.