Star Wars has teased the greatest mystery from Star Wars Rebels. The spiritual successor to The Clone Wars animated series, Star Wars Rebels was set shortly before the events of the original movie. It starred a small group of Rebels who were battling to free the planet Lothal, and who swiftly found themselves becoming part of a much larger movement.
One of those Rebels was a Jedi, Kanan, who had survived Order 66 thanks to the actions of a self-sacrificial master. Another was his young apprentice, Ezra Bridger, a teenager who had tremendous Force potential. Viewers naturally expected Kanan and Ezra's stories to end in tragedy, given they wouldn't be around at the time of Luke Skywalker. In a smart twist, those expectations were only half-fulfilled; while Kanan did die, Ezra survived, albeit vanishing into the Unknown Regions in order to defeat Grand Admiral Thrawn. The Rebels season 6 finale featured a flash-forward scene after Return of the Jedi, with Sabine and Ahsoka heading to the Unknown Regions to locate him.
The latest Star Wars tie-in novel, Alexander Freed's Shadow Fall, teases the mystery of Ezra's disappearance. This, too, is set shortly after Return of the Jedi, likely concurrent with Ahsoka and Sabine's mission to the Unknown Regions, but it tells the story of former Rebel - now New Republic - general Hera Syndulla. In one key scene, Hera looks at the continuing conflict with the Empire, and wishes she had a Jedi Knight with her. Fellow Republic leaders notice a strangely wistful expression on Hera's face, which makes sense; Hera was Kanan's lover, and bore his child Jacen, and the missing Ezra Bridger was one of her closest friends.
The Star Wars Rebels season 4 finale aired in January 2018, but Lucasfilm is yet to give even a hint of Ezra's fate. It's possible further clues will be provided in The Mandalorian season 2, which features Rosario Dawson playing a live-action Ahsoka Tano several years after these events. Although there's no reason Ahsoka should tell a random Mandalorian bounty hunter what she's been up to all these years, there are sure to be further clues.
Clearly Hera herself never truly stopped grieving for the loss of Kanan and the absence of her friend, Ezra. The crew of the Ghost were a tightly-knit team, and Hera played something of a mother-figure to Ezra - which no doubt trained her for later bringing up a Force-sensitive child. Hopefully, Lucasfilm is saving the story of Ezra's return for later, and it will see the surviving Ghost crew reunited.