Danny Phantom x Captain Marvel idea!!!
I've seen fic where Billy gets adopted by Danny, or Phantom sees Captain Marvel and his mind yells, "THAT IS A CHILD", etc.
But what if he treats Billy like a baby? Imagine ancient (may be king or not) spirit Danny, who watched his family die, just takes a look at Billy (in his Marvel/Adult form) and his core just latches on to be his mama.
As in, he treats him like an ACTUAL baby! He puts him in a crib, Reads him bed time stories or sings him a lullaby, Burps him after making him drink bottled milk, Carries him around like a child, etc. All while he is in his Phantom form (it doesn't age) and Billy is in his Captain Marvel form.
Worst Best part?? Billy is very much fine with this. No. In fact, he is enjoying this. It feels warm, fuzzy, and fluffy all around his heart. This ghost is his mama now. He feels so content, safe, and wholesome. You cannot take this away from me, Batman! No, I don't care if we look weird, Superman, Mama says it's nap time! Shhh, Mama is talking to me (using baby talk lol). Gimme a sec, Wonder Woman.