What if, during the final revival of Paganism that started to take place during the crises and invasions after the end of Theodosius I's reign, a strong pagan emperor came to power in Rome and reversed the ban instituted by previous emperors? Could paganism have been revived?
After Theodosius the Great, it's too late. Heck, even Julian Apostata had his work cut out for him, and I'm not entirely sure it wasn't too late entirely for him, either.
I think in order to see a pagan revival, you're going to need to remove Constantine the Great from the picture, or, at least, make sure that all of his successors are de facto pagans. Constantine's importance cannot be overstated. No, he did not root out paganism, though he turned the tables on paganism towards the end of his reign. But his legalization of Christianity gave the already vibrant Church such a boost, and such a patron, that in the 20+ years he sat on the Roman throne, he pretty much tied up his successors' hands.
A pagan emperor as strong and forceful as Constantine immediately succeeding him might have pulled off revoking Constantine's privileges for the Church, and given the old ways favor again. But it would take something I cannot imagine at the moment, for a full-scale pagan revival.
No, Rome wasn't thoroughly evangelized by Constantine's death. Heck, even at the time of Augustine 100 years later, there were still pagans around.
But you'd need to remove Constantine and his successors in order to make paganism revive and thrive in the Empire. Post-Theodosius is entirely too late.