Latin never went anywhere! Carpe Diem, ad hoc, et al. I love learning more about Latin, and while it would be cool to be as fluent as possible as a speaker, I really love parsing and consuming Latin texts, I learn so much not only about history or religion but also just about our current society and language habits.

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.