My writing work takes a lot of different forms, from working for groups who have a very specific song and concept, to open-ended compositional commissions, to things I write for particular pedagogical reasons.
My favorite arranging, though, is when I can insert more of my own musical perspective into the work. In this case, the process for selecting a song comes down to this question: can I add something?
I personally don’t see much I can add to Stevie Wonder’s Signed, Sealed, Delivered, though I’m certain another arranger might have different feelings. Gene Puerling had no trouble adding something to Stevie’s You Are The Sunshine of My Life.
To stick with Stevie, I can imagine writing an arrangement of You And I that would allow me to add something to the song in a way I couldn’t with Signed, Sealed, Delivered.
It’s important to have the functional skills to be able to arrange virtually any song, should the need arise. But it takes discernment to look at a list of songs, and find the right ones that will allow you to add your own voice.