I don't know how much of his story is covered in Rebels, but I could really go for a Captain Rex cartoon, book, or even a stand-alone movie. He's a great character, and it'd be fascinating to see his post-Clone Wars story told.
Edit: Episode VIII rumor/fakery (I hear it may actually be placeholder art?):
PLAY |PS4: Tales of Berseria, | PSVR Super Hot, FF Monsters of the Deep | Switch Xenoblade Chronicles 2 WATCH | Westworld S2, Jessica Jones S2 READ | Andrzej Sapkowski - Sword of Destiny, The Hunter - Andrew Reid