"Traditional" pagan religions lacked the structure and organization to spread outside their home culture. Most even lacked the concept of conversion. But a reformed pagan systems borrowing influences and philosophies from a defeated Christianism and/or Zoroastrianism may succeed: Mithraism, Sol Invictus, Cult of Isis, Cult of Cybele...
Early Christianism itself was for a long time a heavily fractured faith, with dozens of lines of interpretations. If it wasn't for Imperial-backed initiatives like the Council of Nicea to define an official version of the dogma for all to refer to, it may have stayed so. Smaller and smaller competing, dissident sects migrating over Europe and Africa, dissolving into local folks beliefs and giving birth to entirely new religions: churches of Wotan or Isis, infused with Christian-derived concepts such as 'soul-saving' conversion/baptism, non-henotheism/exclusive worship, proselytism, ordained clergy, written scriptures, etc.