One idea could be a crossover with 'Doctor Who'. The TARDIS could land in Hill Valley, and the Doctor could team up with Marty McFly. They could use the time - traveling capabilities of both the DeLorean and the TARDIS to fix some major time - related disasters across different universes.
First, decide which two (or more) universes you want to cross over. For example, if it's 'Back to the Future' and 'The Terminator', think about how the time - travel concepts in both will interact. Then, create a character - driven plot. Maybe the Terminator is sent back in time to Hill Valley to stop Marty from changing the future, but Marty has the DeLorean to outwit him.
One idea could be a crossover with 'Doctor Who'. The DeLorean could time - travel to a universe where the Doctor exists, and they team up to fix a time rift that's threatening both their timelines. Maybe Marty and the Doctor have to figure out how to combine the technology of the TARDIS and the DeLorean to set things right.
In a crossover with 'The Matrix', Neo would be an interesting addition. He could use his skills in the virtual world to help Marty and Doc Brown understand and manipulate the digital aspects of time - travel technology that might be affected by the Matrix. His ability to see the code could potentially help them fix glitches in the DeLorean's time circuits.
Well, the back story is centered around Doc Brown's brilliant yet eccentric inventions. His time machine is at the core. Marty accidentally uses it and is thrust into the past. There, he encounters his young parents and has to navigate the social norms and events of 1955. It's a story that blends time travel, family, and the changing eras in a really fun and engaging way.
One of the main themes is time - travel. It drives the whole plot as Marty travels back in time and has to figure out how to return to his own time. Another theme is family. Marty gets to see his parents in a different light when he goes back to the past and realizes the importance of family relationships. Also, the idea of self - improvement is there, as Marty's actions in the past end up changing his family's future for the better.
The back story of future kings could involve their upbringing in noble families. They might have been trained in leadership, combat, and diplomacy from a young age. For example, they could have been tutored by the wisest scholars in the land, learning about history, law, and different cultures.
It could be about exploring what a 'normal' future in 2015 would look like in the context of the 'Back to the Future' universe. Maybe it focuses on how the characters adapt to the changes that were predicted in the original story but with a more down - to - earth or normal take.
One great 'Back to the Future' fanfic is 'A Different Future'. It explores what would happen if Marty didn't go back in time successfully to fix things. Another interesting one is 'The Time Travelers' Allies' which focuses on new characters who team up with Marty and Doc. And 'Back to the Roots' is also good as it delves deeper into the relationship between Doc and his younger self.