If Luke were recast, the popular opinion is Sebastian Stan. As for Thrawn, Lars Mikkelsen voiced him in Rebels, and could look the part easily. BUT I agree with you about introducing parts of the universe to see how well it does. So I doubt we’ll get Ashoka again, in the Mandalorian. This is the Mandalorian’s story, and Gorgu’s story. This isn’t Rebels, Part 2. In its usual Star Wars way, it’s a little bit of fan service to bring her in, and tell us where we are in the narrative - last we saw of Ashoka was at the end of Jedi (per Rebels flash forward), where she was going to go look for Thrawn and Ezra. So, 5 years later, she is still on that quest. That’s throwing a bone to us. Everyone is very concerned that we’re going to see Luke or Ezra answer the call at the Jedi Temple, but Ashoka said that it’s Gorgu’s choice to make if he even wants to make the call. Maybe he gets there and decides he’d rather stay with Mando, and instead we get their continuing adventures.