Numbers are for illustration only -- if the next pope decides to skip a number or repeat a number for whatever reason, it will resolve as the same name anyway (e.g. "John XXIII" or "John XXV" or even "John MMXXIII" would still resolve as John XXIV). In the event that the Catholic Church starts allowing female priests, a female name with an obvious male equivalent in this list would resolve as it (e.g. "Paula" would resolve as Paul VII).
I was thinking of creating a market like this myself; but I'd have split "other" into a few distinct categories:
A new, never before used name (e.g. Anthony, Ignatius)
Peter II (this would be a big scandal, probably)
Linus II (Pope Linus was Peter's direct successor, so it'd probably still be scandalous)
the same name as one of those obscure Medieval popes (Gelasius, Zosymus, Hormisdas)
Some other "the second" or "the third" name
@DanielFilan I think that's too tightly coupled with the name. I think they technically can pick the number they want (Pope Diabolus DCLXVI lol), but in practice I expect them to follow the existing numbering. So a 2 will most likely be Francis, 3 John Paul, 24 John and so on.
Now, maybe there could be a market on which century it was that the most recent pope chose that same name.
@BrunoParga Sure - it just nicely collapses "brand new names" into one bucket, as well as "existing but obscure names".
@evergreenemily @OnixarLilen @Odoacre @DylanSlagh I've updated the description -- let me know if you think these two scenarios have a non-negligible probability so that you would have bet any differently if they had been mentioned in the description all along.