It could be her husband, Shining Armor. In the lore, they are in a relationship, so it's very likely that he would be the father. This would also bring in elements of their relationship and how they support each other during this time.
Shining Armor, her husband, would surely be a main character. He could be shown as a very supportive and protective partner. He might be by her side during doctor visits, making sure she is comfortable and taking care of all her needs.
A possible plot is that an old enemy tries to take advantage of her pregnancy. Maybe they think she's vulnerable in this state. However, Princess Cadence proves them wrong by using her inner strength and the support of her friends and family. Also, there could be a plot about her sharing her pregnancy experiences with other female characters in the kingdom, creating a bond among them.
It could be another powerful magical being. Maybe a long - lost love from another dimension who has the power to traverse realms and had a brief but passionate encounter with Celestia.
In a fanfiction, Cadence's pregnancy could bring out different emotions. Shining Armor, her husband, would be overjoyed but also a bit nervous. Cadence herself might face some challenges. Maybe there are some old Crystal Pony traditions related to pregnancy that she has to follow. And the other ponies in Equestria would be sending their well - wishes and gifts from all over the land.
It could be Rick. Given their close relationship and shared experiences, it would make sense for him to be the father. Their bond has been strong throughout the series, and this would add another layer to their relationship.
It could be her long - time boyfriend. They've been in a loving relationship for years, and this pregnancy is the next step in their journey together.
It could also be someone new, a character created specifically for the fanfiction. Maybe a powerful new demigod who catches Annabeth's eye, and their relationship leads to the pregnancy.
It could be a long - lost love from Mavis's past. Maybe someone she thought she'd never see again but who re - enters her life right before she discovers she's pregnant.
It could be another powerful magical being in her realm. Maybe a long - time ally or a mysterious figure that has been in the background of the story's world. This would add an element of mystery and intrigue to the fanfiction.
In some fanfictions, the father could be a human character. This would add an interesting dynamic as it would involve dealing with the differences between a human and a Digimon in a relationship and during pregnancy. It could also lead to a story about acceptance and dealing with the unconventional nature of such a union.