At the risk of sounding creatively bankrupt I wouldn’t mind if the third film uses the same songs from the first two films in a different context. Those songs are unbelievably iconic and are so popular because we listen to them again, and again, and again.
So give us a third rendition of Dancing Queen, another group version of Mamma Mia, a striped back acoustic version of Voulez Vouz, a duet version of Winner Takes It All, etc.
"Charlotte, we're Jewish"