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.
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.
In a fanfic, their relationship could be developed by having Celestia make sacrifices for Nightmare Moon. This shows that she truly cares about her sister, even in her Nightmare Moon form. For instance, Celestia could give up some of her magic to try and heal Nightmare Moon. Nightmare Moon, in turn, might be initially suspicious but then start to question her own hatred towards Celestia. You can also have them interact with the other ponies in Equestria. Seeing how Celestia is loved by the ponies and how they fear Nightmare Moon can make Nightmare Moon re - evaluate her relationship with Celestia and Equestria as a whole.
One possible plot could be about the main character growing up under Celestia's care in Equestria. They could learn about magic and friendship directly from her, and face challenges together like dealing with threats to the kingdom. The story might also include some heart - warming moments of Celestia as a mother figure, like tucking the character into bed at night and telling stories about the old days of Equestria.
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.
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.
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.
One great MLP Princess Celestia fanfic is 'The Sun's Daughter'. It delves deep into her backstory and her relationship with Luna. It shows a more vulnerable side of Celestia which is really interesting.
I'm not sure specifically as there could be many different 'My Little Pony' fanfics where Celestia dies. It could be about the reactions of the other ponies in Equestria, like Twilight Sparkle having to step up in a new way without Celestia's guidance. Maybe it also explores how the balance of day and night is affected if Celestia is no longer there to control the sun.
One interesting aspect of MLP Celestia and human fanfic is the exploration of different relationships. It could be a friendship between a human and Celestia. The human might bring new perspectives from the human world, like different forms of art or technology concepts that are alien to Equestria. This can lead to some really fun and creative storylines.
A fanfic titled 'Luna's Redemption in Celestia's Eyes' is quite interesting. It focuses on Luna's journey towards redemption from Celestia's perspective. It shows how Celestia has always believed in Luna, even during her darkest times. The story is filled with emotional moments that really tug at the heartstrings.