It could be Dumbledore in his younger days. Dudley would be really shocked to meet the great wizard when he was still young and full of energy. Dumbledore's wisdom and power would be a complete contrast to what Dudley is used to in his Muggle life.
Maybe he meets a young Snape. Snape's dark and brooding nature would be very different from anyone Dudley has met in his own world. And Snape's complex relationship with Harry's mother could add an interesting layer to Dudley's time - travel experience. Also, the fact that Snape is so deeply involved in the wizarding world's politics and secrets would be mind - boggling for Dudley.
I think it would be really unexpected if Dudley met a house - elf from a long time ago. House - elves have such a unique culture and way of life in the wizarding world. Dudley, coming from a Muggle family, would have no idea about these magical creatures. The house - elf could show Dudley some hidden corners of the wizarding world's past, and Dudley would have to deal with their strange loyalty and magic - based servitude.
Boruto could be an unexpected ally. Maybe he somehow gets caught up in the time - travel event with Sarada. They have a strong bond in their own time, and that would carry over. They could work together to face the challenges of the different time periods, like when they encounter powerful and dangerous ninjas from the past or future.
Poison Ivy could be her ally during time travel. Since they have a strong bond in their own time, Ivy could follow Harley through time accidentally. And Ivy's connection to nature might be useful in different time periods. For instance, in a pre - industrial Gotham, Ivy could use her powers to communicate with the natural world to find a way to get them back to their own time.
Maybe Rumplestiltskin. Even though they have a complicated history, in a different time period, they might find common ground. He could be interested in her time - traveling abilities and offer his own brand of help in exchange for something from her future knowledge.
One interesting plot idea could be Dudley traveling back in time to when his parents first got married. He could see how his family's attitudes towards magic and the Potters were formed. Another could be him traveling forward in time to see the impact of Harry Potter's actions on the wizarding world and how it indirectly affects the Muggle world he lives in. Maybe he travels to a time when a major wizard - Muggle conflict was about to start and he has to use his new - found knowledge of both worlds to prevent it.
His best friend, Boruto, could potentially be an ally. Even if Boruto doesn't physically travel with him, Shikadai could remember Boruto's advice and use it during his adventures. Boruto's positive and adventurous attitude might inspire Shikadai in difficult situations.
Her older brother Albus could be her sidekick. They have a close bond and Albus is likely to be curious and adventurous enough to join Lily Luna on her time - traveling escapades. Together, they could face the challenges of different time periods and use their combined knowledge of magic to solve problems.
Well, in his time travels, Rock could encounter other time - displaced robots. These robots, who are also out of their normal time frames, could become his allies. They might have different perspectives on the Mega Man universe and could offer different strategies. Additionally, there could be some hidden characters or entities in the Mega Man lore that only show up during time travel events and become Rock's allies. For example, a wise old robot that has been observing the timeline from the shadows and decides to help Rock.
Jiraiya would be a great character for Naruto to interact with. If Naruto time - travels and meets Jiraiya, Jiraiya would probably be curious about Naruto's strange situation. He might try to write a new book based on Naruto's time - travel in stone. And they could have some really funny exchanges as Naruto tries to explain modern ninja concepts to Jiraiya while being stuck in a stone - like state.
Darth Vader could be an unexpected ally in a time travel scenario. If Luke travels to a time when Vader was still Anakin Skywalker and on the verge of turning to the dark side, Luke could try to prevent his fall. In the process, Anakin might become an ally, using his skills as a powerful Jedi to help Luke in whatever mission he has in that time period.
Luffy would be really interesting. His carefree and adventurous nature would make his reactions to different time periods really fun to explore. He might try to befriend everyone he meets in the past or future, regardless of the consequences. His simple view of things could also lead to some unexpected changes in the timeline. For example, he might accidentally start a new pirate crew in the past or inspire a revolution in the future just by being himself.