Yeah, I could definitely see a young Leia showing up in later seasons possibly, which would mean she would continue to get older and get closer to the right age. The only reason I think they would not want to cast her would be because they might want a less recognizable actress to play Leia, so we just see “Leia” rather than Eleven. Though honestly with the right costume and hairdo, I think she could pull it off.
For me, I am curious of what we will see of Cassian’s characterization. Will we be introduced to him when he is already in the Rebellion, or will we get to see how he got recruited? Will we see him as a child during the Clone Wars? Maybe we will get flashbacks to his childhood and/or recruitment, who knows. I definitely think though that we see Cassian grow to become weighed down by the morally questionable actions he must take for the Rebellion. There is a lot of potential I think with the idea of him having this noble idea of fighting the good fight when he initially joins or whenever he becomes an intelligence officer, to him slowly begin to realize how the line between right and wrong is blurred during war.
I could picture a scenario where he presented with a difficult choice during a mission, and he ends up going with the “moral” choice, but the choice to follow his moral code ends up having dire consequences. Maybe by choosing to not kill one person ends up getting other members of the Rebellion killed.