You could write a fanfiction where the '100 days' are not consecutive. Maybe there are gaps in between due to some unforeseen circumstances like a war or a political upheaval. This would create a more complex and interesting story arc as they try to reconnect during those breaks.
Another plot could be that the 'kiss to my prince' is part of a bet or a game in a fictional kingdom. The girl who kisses the prince then has to face the consequences of this action, like dealing with the jealousy of other noble ladies or having to prove her worth to the prince and the court. As they interact more, they start to have real feelings for each other.
Another interesting angle could be a different group of survivors during the 30 days. How they cope, what strategies they use to stay alive that are different from the ones in the original story.
Another interesting angle is to create an alternate universe for 'The Black Prince'. In this universe, he could have different abilities or face different challenges. For instance, if he had the power of magic in this AU, how would it change his story and his role in the world?
One idea could be to cross Dog Days with a popular magical girl anime. For example, the characters from Dog Days could meet and team up with the Sailor Scouts. They could combine their powers to fight a new, powerful enemy that threatens both their worlds. This would bring a fresh dynamic as the Dog Days characters' combat styles are quite different from the magical attacks of the Sailor Scouts.
One idea could be exploring what happens if the infected start to regain some of their humanity. Maybe a group of survivors discovers a way to reverse the infection slightly in some of the infected, leading to a tense co - existence situation.
One idea could be exploring the backstories of the characters more deeply. Maybe delve into their lives before the events of the original story. Another could be creating an alternate ending where different choices lead to a completely different outcome for the characters. For example, if a character who was a villain in the original story could be redeemed through different circumstances.
One idea could be a group of friends going on a 2 - day 1 - night camping trip. They encounter various challenges like bad weather and getting lost, but also have a lot of fun moments around the campfire sharing stories. Another could be a couple's romantic getaway for 2 days and 1 night. They visit beautiful places, have candle - lit dinners, and strengthen their relationship.
One idea could be to create a story where a new character from our world is suddenly transported to the world of Dog Days. They could have unique abilities that disrupt the normal balance in the land, leading to new adventures and friendships. For example, they might be able to control an element that doesn't exist in Dog Days, like electricity. Another plot could revolve around a character from Dog Days visiting our world and experiencing modern technology for the first time, with all the confusion and hilarity that would entail.
One idea could be exploring the Dark Prince's backstory more in - depth. Maybe he had a love interest before he became the Dark Prince that could be reintroduced into his life, causing an internal struggle between his dark side and the remnants of his former self.
One idea could be exploring the backstory of the Prince's family in more detail. Maybe there are secrets about his ancestors that are yet to be discovered.