Why not bring back Latin? The alphabet is already in wide use, many languages evolved from it making it easy-ish for them to learn, and Latin was already in wide use as the lingua franca for academia and the church up until the 1700s.
The answer with Latin is obviously the cases, imho Spanish would be my vote for a lingua franca – simple, phonetic, sounds beautiful with any accent sung or spoken, and already has massive influence and history.