Another plot could be that Chris gets sent to the future by accident. In the future, the world is completely overrun by the creatures from the bio - horror universe. He then has to find a way to travel back to his own time while also gathering knowledge about how to defeat these future threats in his present. For example, he might discover new weapons or weaknesses of the enemies.
A plot could involve Wesker faking his death and then secretly observing Chris. As Chris continues to fight against bio - threats, Wesker starts to have a change of heart and decides to help him from the shadows. Eventually, Chris discovers Wesker's presence and they have to decide whether to continue their enmity or work together. It could also be a fic where they are both trapped in a virtual reality created by an evil organization, and they have to rely on each other to escape.
Leon Kennedy could also be an ally. He has his own set of skills and experiences in dealing with dangerous situations. In a time - travel scenario, Leon might be able to provide different perspectives and resources. For example, if Chris travels to a time where Leon has already been, Leon could give him valuable information about the local threats and how to deal with them.
They could time - travel during a great war in the past. Kakashi and Sakura might find themselves in the middle of battles they only read about in history books. They could use their modern - day skills to change the outcome of certain battles. This would also have a huge impact on the present - day Naruto world as they know it.
One interesting plot idea could be that Harry travels back in time and meets Lily before she has him. He could try to prevent her death by warning her about Voldemort. Another could be that Lily time - travels to the future and sees the world after Voldemort's defeat, getting to know the man her son has become. Or perhaps they both travel to a different time altogether, like the time of the founders of Hogwarts, and have to adapt to that era while also dealing with their own relationship and the knowledge they bring from their own times.
Another interesting plot is Luke traveling to the future. He discovers a galaxy in chaos due to a new, powerful dark force. He has to adapt to new technologies and new forms of the Force while also finding a way to defeat this new threat. He might meet descendants of his friends and foes, which would add an interesting dynamic.
Maybe they time travel to a point where a major battle was lost in the past. Kakashi and Team 7 could change the outcome of that battle. For example, if they go back to the battle against the Nine - Tailed Fox when it first attacked Konoha. They might be able to prevent some of the destruction and save more lives. Or they could travel to a time when a powerful ninja was wrongly exiled, and they work to clear that ninja's name and change the course of history.
Perhaps they travel to a time in the past where a powerful and dangerous jutsu was first created. They have to stop the jutsu from being misused in the future. Along the way, they face challenges like dealing with the original creator who might not want to give up on his creation, and also face other ninja who want to steal the jutsu for their own evil purposes.
One idea could be that they travel back to a time when the founding families first made the deal with the vampires. They could try to change the terms to prevent all the future chaos. Another could be traveling to a time when they first met, but with their current knowledge and feelings, and seeing how that alters their relationship from the start.
One idea could be that Lily Luna travels back in time to the Marauder's era. She could interact with James, Sirius, Remus and Peter, and perhaps change some small events that have a big impact on the future. For example, she might prevent a prank from going too far and in turn, change the relationship dynamics among the group.
One idea could be Natasha going back in time to prevent a major event that led to the loss of many lives. For example, she could stop a certain villain from getting a powerful weapon in the past. This would create a whole new timeline and she would have to deal with the consequences of changing history.