Boba Fett & Captain Rex
Before Din Djarin, Boba Fett was the most famous Mandalorian out there. We know he survived the Sarlacc pit after the events of Return of the Jedi, which means the chances of him showing up are pretty good. If the rumors are true and Jango Fett actor Temuera Morrison is going to pop up on the series, it would only make sense that he also played Captain Rex, who we last heard was fighting with the Rebellion at the Battle of Endor. Considering where these clones left off, there are plenty of reasons for them to appear on The Mandalorian.