iō Philadelphiam! (Yo Philly!)
iō Philadelphias! (Yo Phillies!)
iō Philadelphenos! (Yo Philly (guys)!)
According to The Bantam New College Latin & English Dictionary, Revised Edition (Third Edition, John Traupman, 2007), iō is an interjection "(expressing joy) ho! hurray!" or "(in a sudden call) yo!"
Dennis felt that "'yo' ... seems natural in Philly." He further explains that this cheer "uses the exclamatory accusative. Philadelphenus,-a, -um is the adjective used in Latin for the people of ancient Philadelphia. This last one could refer to the players or the fans." Dennis recommends the second option, stating, "I think I like 'iō Philadelphias!' the best."
(For those of you who want to know precisely what an Exclamatory Accusative is, George Lane's Latin Grammar for Schools and Colleges explains that this type of accusative is used to call attention to something. It can occur with or without an interjection. Common interjections used with the Exclamatory Accusative are: heu, eheu, pro, iō, etc.)