For some reason I can only see Twilight as Zazu because of the way see overreacts, and desires to make everything situation perfect just like him. She calculating, organized and, every chance she gets she is always by Celestia's side just like Zazu is always by Mufasa side when he gets the chance.
Twilight is Spike's guardian just as Zazu is Simba's guardian when the parents aren't around. Plus I feel that Twilight is too responsible and well behave to play as Simba, its much easier to picture Spike as Simba than Twilight, seeing him being mischievous while flaunting, and playfully abusing his title as future king at first. Then growing into a mature and powerful king after realizing his duties to his subject at the second half.
Plus with that whole scene in Secret of my Excess where Spike compares his claw to the giant claw he made in the ground from his growth just clinches it. It reminded me how Simba compared his paw print to his father's own, so between Twilight and Spike, there's no one better to play as Simba than Spike (except Simba himself lol).
On a side note wouldn't Prince Blueblood make the perfect Scar