I agree there though I'm not too hopeful on Nala while I've liked her in every movie not a whole bunch has been done with her. Simba though he's the main character of the franchise I mean yeah his role was reduced in one and half but he was still a pretty big player there so I don't really see that changing Simba will probably have a big role.
I'm assuming a supporting arc in the show will be the dynamic between Simba and Kion which I'm not sure I want we've had a lot of father and child relationships I would rather Simba and Kiara be close while Kion is more at odds with his father and is much closer to his mother. Nala would actually know more about defending a kingdom than Simba would have Nala be kind of a mentor to Kion giving her a bigger role than she usually is not to mention allowing us to see what kind of a parent Nala is.
This is just what I would prefer it would create an intriguing dynamic with the show you continue expanding on the bound between Simba and Kiara while showing Kion's relationship with Nala throw in a little conflict between Kion and Simba and I feel you have a good family arc.