Ideas to include
The proposal soundtrack
Play it right before you ask, or right after they say yes. Tell us which, because it changes where the song has to land.
The thing you never said
The gratitude, the apology, the admission. Write it plainly in the brief and let the song do the hard part of saying it.
The ordinary Tuesday
No anniversary, no reason, no warning. A song that shows up because you meant it, not because a date required it.