4. The classic fade out
Whitney Houston's “I Wanna Dance With Somebody,” Wham!'s “Wake Me Up Before You Go-Go,” and Billy Joel's “Uptown Girl” are just a few examples of incredible songs that ended with a fade out. It's a true classic that despite its age is still a great choice. Usually, you just keep repeating the chorus (or the hook) over and over again, improvising more as you go along, all while the music fades out. This gives the song the feeling that it never really ends.
So take a page from Natasha Bedingfield's “Unwritten”, which also ends this way—if the topic of your song revolves around any sort of untold, unwritten, unresolved, unfinished business, then a classic fade out may be the perfect way to go to musically match the lack of resolution in your lyrics.