That would become “Cyril” when conventionally rendered into English. Has been used by Popes of the Coptic Church.
Sixtus the Sixth would be fun to make anchorpeople stumble when reporting. Plus it’s just one six short of freaking out a lot of American fundies.
Maybe go Old Testament on the flock and pick Pope Ezekiel or Pope Isaiah and tell them really big s#!t is coming down if they don’t get their act together.
Maybe seek a yet-unused one. Lando (913-914AD) was the last nonrepeating Pope, and was the last “new” name until John Paul I chose to do a compound name. I like the idea of seconding some long-ignored one-shot Papal name as a way to correct the neglect, but at the same time I’d feel odd taking away his uniqueness (“Hey, he is THE Pope Conon”).
Many “big” names in the Canon remain unused - no Peter, we can understand, but if I’m counting right there’s also: No Joseph, Anthony, Dominic, Ignatius or Bernard; no Louis or Didacus(Diego). There are 23 Johns, 6 Pauls, a Mark and a Stephen, but not one Matthew or Luke, no James, Philip, Andrew, or Bartholomew.