1. Kill off your main character's romantic partner. Have them go through the pain, the grieving of the loss of a loved one, only to rise up and achieve the main aim of the novel (fantasy, action, sci-fi, whatever) on their own. They then find out that their partner didn't actually die, and [insert angst here]. They don't end up together.
2. Write a gothic novel focusing on modern day taboos. They can either be topical - coronavirus, Trump, murder hornets, australian wildfires, etcetera, or long term - advancements in technology, global warming. Write something that is truly thrilling - without resorting to the old techniques of physical monsters or the majority of gothic conventions.
3. Write a novel where there is a queer friends-to-lovers or eventual enemies-to-friends-to-lovers, without queerbaiting or killing off one character in the end.
4. Write a novel that's happened just after the dramatic overthowing of the government, and detail how the people who overthrew the government have to deal with the next people who come into power, who essentially are just recreating the government, except with their own people. They come from the minority group (I.e. the country has a split between two different devisions where there is approximately a 40/60 split, and it's the people in the 40 who are now in charge), and start making life hell for the 60, turning the rules that the 60% had on the 40%. Write about the chaos, the bloodshed, the terrorism, and how they eventually find peace (or don't, and the country just descends into an anarchist state).
Challenge: Don't make the divide male/female, or about colour.
5. Write a piece of metafiction, where the reader thinks that they are reading a book about global warming, or some dystopian advancement in technology, that describes how society went from better to worse (despite seeming to go better at the start), but they are actually reading an account (a diary of sorts, though not presented this way) written by the main character themselves, to dictate their side of the events and to clear away the rumours that have been sparked about them.
6. A person believes that they are talking to God, and that God is telling them to kill bad people all around the world, irregardless of their race/gender/sexuality/etcetera. Make them preach about the gospel and the 'good life', and believe that they can bless people with forgiveness. Then, 1 of 2 things happens.
1. They are caught by the police, the police find out that the person has schizophrenia.
Challenge: He doesn't have schizophrenia.
2. God (or another deity) appears and tells them what they (the deity) thinks about what the man is doing.
Challenge: The voice isn't actually from the God the person is preaching about, but someone/something else.
7. Your main character is the last student of a dying subject (for example, Latin). They start a campaign to reinstate the subject in more schools, and secretly come across a hushed up plot.
8. A spy thriller, except the spy can't actually perform any parkour level acrobatics, and is unintentionally part of a plot to triple cross their country, until they find out that they were actually a sleeper agent for another country, a fact that they themselves were not aware of.
9. Dragons are real. You know, because you've just found one sitting in your back garden that remarked indignantly stating so when you asked it what it was.
And then the dragon tells you it's the rightful heir to the throne, and you help it in its quest to get back the throne, and don't wonder why there is so much opposition until you are told that the dragon you're trying to help is an oppresive scheming ruler who killed most of his subjects for the fun of it.
10. Earth is an exotic destination that is part of a hit reality show (Or pick a better name). Each period of time several (criminal) aliens get placed on earth in various different locations in human skins (in the form of someone waking up from a coworker). They have to survive the equivalent of 1 month on earth without being captured. If they manage to do so, they can go back to their home planet and re-habilitate themselves. If they don't, then they are either caught by the human police and never seen again or killed by the alien race themselves.
Hope that's enough for now! :)