AND!!!! (I'm obsessed with fanon Leon, please bear with me) How Leon adjusts to Arthur's new reign.
Leon was always loyal to Uther. Yet, Arthur changed a lot of how things were done. Most of which Leon agreed with, but definitely some he didn't. It must have been difficult for him to do the things Uther required of a knight.
We've seen how Arthur felt with the ghost in the well when it was revealed how he couldn't stop his knights from slaughtering people in that camp.
How do you think Leon felt? Was he there? Was he participating? Did he do it out of loyalty or survival? Has he been leading raids like this? Does he feel guilt? Has he been forgiven?
I kinda took these questions to write "Uther's knight", of course with the fanon interpretation of Leon as an immortal. because DUH.
My point is, there is so much we can make of these characters. Percival, the gentle giant who barely speaks is a very good example, as OP said.
Gwaine was cut short as well, seeing as his confrontation with the king (of Caerlon) who had his father die for him and then left his mother in poverty was killed right in front of him (by Arthur.) we never saw him grief nor rejoice. And we haven't seen him interact with Annis, which would have been AWESOME!!!
And ... You know, as much as I hate Agravaine, I'd love to know more about his relationship to Uther. Where was he (canonically) during the first seasons? How much did Uther trust him? How was he there for Arthur that he'd be trusted to unconditionally.
What about Gwen's and Elian's knowledge of blacksmithing? Where was Elian anyway? What did he learn? Whom did he meet. Who did he learn to fight for?
What's the consequences of Aridian's witchhunts in other parts of the kingdom?
Did any of the council members ever speak about Ygraine to Arthur after it was safe to do so?
Who taught Gaius to be a physician? He never mentioned his mentor's name.
And was identity fraud the only crime Lancelot ever committed?
I want to know. I want to know everything.