Some fandom songs I have thought of (Don't kill me if the lyrics don't exactly fit with each character):

"Demons" by Imagine Dragons- Nico Di Angelo (It scares me how much this song sounds like it was written by him)

"If This Was A Movie" by Taylor Swift- Percebeth in between TLH and MoA 

"Perfect" by P!nk- Leo Valdez to Hazel Levesque (sorry if the lyrics aren't exact but I thought of it)

"Unconditionally" by Katy Perry- Hazel and Nico (brotherly and sisterly love of course)