The official hymn for the year of Mercy has been chosen and promulgated. You can get the English music and words version here.
It is Latin chorus "misericordes sicut Pater", with an recited sentence and then another repeated refrain "in aeternum misericordis eius". Music by Paul Inwood and text Eugenio Costa.
I have to say that it is fine. It is all a bit too Taize in feel (so with neither the glory of plainchant melodics, nor the singability of metrical hymnody) but I can see it being used effectively in large liturgical settings. You still need a choir which means that it will actually be difficult for smaller churches to use, but the chorus is catchy in the same way that Taize can envelop you in the duvet of meditation.
It is infinitely better than that ghastly "Cry Jubilee" thing that we had for the year 2000.