It's deeper than Fate reference. Mei turned into Herrscherr to protect Kiana (By taking over Gem of Conquest, which in turn became Core of Thunder after mixing with Gem of Desire from Wendy). Now that Kiana has turned into Herrscherr of Flamescion who has complete control of her powers and not plagued by Sirin/HoV, the consequences of Mei's action has become bygones.
In meta sense, putting HoF as leader will greatly benefit HoT (She gives Element damage boost, which is often needed to finish Lotus tier Abyss.)
It's less referencing Fate, and more like redeeming Mei from her actions.
Rathurue said:
Now that Kiana has turned into Herrscherr of Flamescion who has complete control of her powers and not plagued by Sirin/HoV, the consequences of Mei's action has become bygones.
And given the main story are slowly but surely reaching to the Post-Honkai Odyssey story, Mei's path of redemption and atonement is inevitable. If the situation calls, Mei has to defect from the World Serpent. She already choose this way, and it's time for her to make amends with Kiana and her friends.