A great idea for a novel could be a love story that spans different lifetimes. The two lovers keep finding each other in different historical periods but are always faced with some obstacle that keeps them apart until they finally break the cycle. Another idea is a story about a journalist who stumbles upon a conspiracy that reaches the highest levels of government and has to fight to expose the truth while being hunted down. You could also write a novel about a group of astronauts who are stranded on a distant planet and have to figure out how to survive and get back home.
Well, you could write about a world where dreams are a form of currency. People are born with different levels of 'dream power' and they use it to buy things, gain status, etc. Or perhaps a novel about a person who discovers they have the ability to enter paintings and has to navigate the strange worlds within them. Also, a story about a young wizard who is expelled from a prestigious magic school and has to find his own way to master magic outside the traditional system could be great.
One great idea could be a story set in a post - apocalyptic world where a group of survivors have to build a new society from scratch. Another could be a time - traveling adventure, where the protagonist goes back in time to change a historical event. A third idea might be a mystery novel set in a small, idyllic town that has a dark secret.