In HOO, it is mentioned that Aphrodite told Reyna that 'no demigod shall ever heal your heart'.
Now, skip to TOA, where Apollo is turned into a human.
In the Burning Maze, it is mentioned that Apollo needs help from Bellona's daughter.
Ok, so random thought here.
Apollo is a god/mortal. But he is not, and never will be a demigod.
So, what if Apollo and Reyna fall in love?
Just a little thought I thought I'd get out there.