A possible plot is that they are on a quest to find out more about their family history as the Riddle Twins. Along the way, they encounter various magical artifacts and spells that are related to the Riddle family. They might also have to deal with the Death Eaters who either want to recruit them or eliminate them. Harry could use his Gryffindor qualities to protect Draco at times, and Draco could use his Slytherin knowledge to help them outwit their enemies. This journey could lead to self - discovery for both of them and a new understanding of their place in the wizarding world.
Well, they could be in a situation where they are suspected of following in Voldemort's footsteps just because they are the Riddle Twins. But in reality, they are trying to do good. They could start a secret organization within Hogwarts to teach other students about the importance of using magic for good, regardless of one's family background. They might face opposition from some of the old - fashioned professors and families. However, with their combined skills and the support of their friends like Hermione and Ron, they manage to change the perception of the wizarding world towards them.