Twilight Sparkle would probably be devastated. She looked up to Celestia as a mentor, so she might go into a state of deep mourning at first. But then, she could be motivated to try and fill the void Celestia left, using all the knowledge Celestia had imparted to her.
The villains in MLP might see Celestia's death as an opportunity. Discord, for example, might be tempted to stir up more chaos now that one of the main forces for order is gone. But some of the villains might also realize that without Celestia, Equestria could descend into a state that's not beneficial for anyone, not even them. So, they could be conflicted between causing trouble and trying to help maintain some semblance of order.
Twilight would be devastated. She might go into a state of shock at first. Then she could become very withdrawn, shutting herself away in her library to grieve. Eventually, she would probably turn her grief into a determination to uphold her brother's values and use her magic to protect Equestria even more vigilantly.
The relationship might develop in stages. At first, the character could be in awe of Celestia because of her status as a princess. But as Celestia shows her nurturing side, like making the character's favorite food or comforting them when they're sad, the character starts to see her as a real mom. Then they might start having mother - child traditions, like having a special tea time every week where they talk about their day.
For Celestia, her hyper aspect could be developed by showing her constantly pushing the boundaries of her magic. Maybe she starts off being able to do basic sun - related magic, but as the story progresses, she discovers she can use her hyper magic to create new sun - powered spells. For the futa characters, their development could be about coming to terms with their unique identity in Equestria. They could start off being a bit shy about it and then grow more confident as they face challenges with Celestia.
Well, in the fanfic, Bruce would probably become more withdrawn. He'd brood over Jason's death, questioning his methods and his role as a mentor. Dick would be angry at the world, especially at the villains who might have been involved. Tim, on the other hand, would be more determined to prove that he can be a worthy Robin, but also carry the pain of losing Jason. Alfred would be the one constant, trying to comfort everyone with his wisdom and care. It's a really tragic event for the Batfamily and their reactions would be complex and full of emotions.
Sakura would be devastated. She has known Naruto for a long time and has seen him grow from a loud - mouthed kid to a great ninja. His death would make her question everything. She might become more withdrawn and serious. Hinata would be heartbroken. She had deep feelings for Naruto. She might even lose her will to fight for a while.
Kayano would be completely devastated. Since she had a special bond with Nagisa, she might become withdrawn and filled with rage, vowing to take revenge on whoever was responsible.
Create an interesting plot. It could be a conflict between order (represented by Celestia) and chaos (represented by Discord). Or it could be a story where they have to cooperate against a common enemy. Make sure the plot has a clear beginning, middle, and end. For example, start with Discord causing some minor chaos in Canterlot, Celestia tries to stop him in her usual way but fails. Then a bigger threat emerges that forces them to work together. In the end, they either defeat the threat or find a new balance between order and chaos.
For Celestia, her character could be developed by showing her struggle with her own perfectionism. When dealing with Discord, she might realize that her rigid view of order isn't always the best solution. This could lead to her becoming more flexible and open - minded. For example, she could start to see the beauty in Discord's chaos in some situations.
They can change her appearance. Maybe make her look more menacing, with darker colors in her mane and tail, or a more stern expression. For example, her eyes could be drawn to look cold and unfeeling.
Their relationship can be developed through shared adventures. For example, if they go on a quest to save Equestria from a new threat. During this time, they'll have to rely on each other, which will strengthen their bond.