It’s also New Year’s Eve; if you wanted to focus on that a bit, “Ring Out, Wild Bells” by Stephanie Martin might well be appropriate. It’s available from Leading Note, but I haven’t been able to find a recording. The words are taken from the poem by Tennyson, but I’m not sure if it’s the complete poem or just part of it.
Serious stuff. Repentance is a theme in Isaiah and the Psalm; 1 Corinthians is more encouraging, and the Gospel reading is about keeping awake.
If you’re of a Marian bent you could use the Alma Redemptoris Mater, which is the Marian antiphon used from Advent to Candlemas. There’s a setting by Isabella Leonarda available on CPDL; here’s a Youtube recording of it sung by women, rather than the SATB:
If Advent carol services and processions are more your thing, you could sing Stephanie Martin’s Legend of the Bird, a carol in the form of a conversation with a robin about the return of Christ. It’s available to order from Cypress Choral Music, and here it is on Soundcloud: