It could occur during a time when Harry is on the run from Voldemort. Mary Poppins, being in the right place at the right time, might see Harry in distress. She could then use her magic to create a diversion or shield Harry from Voldemort's attacks. Since Mary Poppins has the ability to enter and exit different realms quite easily, she could whisk Harry away to a safe place, perhaps even a place outside of both the wizarding and Muggle worlds where Voldemort couldn't reach him.
One possible plot could be that Mary Poppins somehow enters the wizarding world of Harry Potter. Maybe she arrives at Hogwarts to teach a special class on manners and propriety, which the Hogwarts students desperately need. She could use her magic umbrella to fly around the castle, much to the amazement of the students. Her unique brand of magic, which is more about making the ordinary extraordinary, could clash or combine with the more traditional spell - casting magic of the wizards.
The plot could start with Harry facing a seemingly insurmountable problem in the wizarding world, like being cornered by a powerful dark wizard. Just when all seems lost, Mary Poppins appears out of nowhere. She might use her magic to confuse the dark wizard and then take Harry away on her flying umbrella.
One way to blend them creatively could be by having Mary Poppins visit Hogwarts. Maybe she could use her magical nanny skills to teach the students some unique lessons, like how to clean up a potion spill with just a snap of her fingers. She could also interact with the magical creatures there, perhaps taming a hippogriff in her own special way.
She could fit in as a mysterious visitor. For example, she might show up during a Quidditch match and use her magic to save a player from a dangerous fall. Her presence would be a shock to everyone as her magic seems so different from what they know. She could then start to interact with the characters, sharing her own brand of magic with them, like teaching Hermione how to make a room clean itself with a simple song.
Magic is a very common theme. Since both 'Harry Potter' and 'Mary Poppins' are set in magical worlds, fanfictions often explore different types of magic. For example, how Mary Poppins' magic of tidying up and her flying umbrella might compare to the spells in 'Harry Potter'.
Yes, you might be able to find such a fanfic on FanFiction.net. But it could take some searching through the various categories and tags.
The link could be anything from character crossovers to sharing similar themes of magic and adventure. It's all up to the imagination of the fanfic writer to make it work.
Sasuke is a highly skilled ninja with incredible speed and combat abilities. If he was transported to the Harry Potter universe, he could use his physical prowess to protect Harry. For example, when Harry is being chased by Death Eaters, Sasuke could use his ninja techniques like the Chidori to fend them off. He could also use his Sharingan to read their movements and anticipate their attacks, thus saving Harry.
First, decide which two universes you want to cross over. For example, if it's Mary Poppins and 'Star Wars', think about how Mary would fit into the Star Wars universe. Then, come up with a central conflict or adventure. Maybe Mary has to use her magic to stop the Empire from taking over a planet. Start with a strong opening that introduces Mary in this new setting, like 'Mary Poppins landed on the sandy planet of Tatooine, her umbrella held tightly in her hand as she surveyed the strange new world.'
A great plot could be that Mary Poppins' magic umbrella is actually a powerful wizarding artifact that has been lost for centuries. When it shows up at Hogwarts, it causes all sorts of chaos as different factions try to get their hands on it. Mary Poppins then has to team up with Harry Potter and his friends to protect it. She could also teach them some new spells and charms along the way. For instance, she might show them how to use the power of song to enhance their magic, like she does in her own world. And her parrot - umbrella handle could have some secret knowledge to share about the history of the wizarding world.