A new kind of magic. Harry as the Master of Death could introduce a different type of magic system compared to the existing magic in Middle - earth. His power over death might allow for some unique plot developments, like bringing back characters who have fallen in the battles of Middle - earth.
Harry Potter's strong sense of justice could influence the Mandalorians. He has always fought against evil, and he could inspire the Mandalorians to take on more noble causes. Maybe he would encourage them to be more inclusive towards other species in the Star Wars universe, just as he has been inclusive towards different magical beings in the wizarding world.
Well, it could be about a story in the Harry Potter universe where someone asks Harry to just bring them flowers. Maybe it's a romantic gesture towards a character, or it could be part of a magical plant - related plot. For example, perhaps the flowers have special properties in the wizarding world and are needed for a spell or to cure an ailment.
Magic research could be one. Bellatrix is very powerful in magic, and Harry has his own unique understanding of magic. They might find common ground in exploring some ancient and powerful spells.
The half a galleon could be part of a magical spell or charm. Harry and Draco might be unknowingly affected by it. They start having dreams about the half a galleon and each other. This makes them curious and they start to seek each other out to figure out what's going on.
Magic. Harry Potter, as the Master of Death, would bring his powerful magic to the RWBY world. His spells could be a new form of combat or defense against the Grimm. For example, the Stunning Spell could be used to immobilize Grimm, giving the RWBY characters an advantage in battle.
Immortal Naruto would bring his superhuman strength and speed, which are results of his ninja training. In the wizarding world, this would make him a force to be reckoned with. His Sage Mode, if he could access it, would give him enhanced senses and more powerful attacks. He could also bring his never - give - up attitude. This would inspire the characters in the Harry Potter world, especially in times of despair. His ability to use chakra to run up walls and across water could also be used in interesting ways in the wizarding world, like during a chase or escape situation.
If the new headmaster was someone like Neville, he might focus on making the school more accessible to students from different backgrounds. He could also introduce more hands - on learning in Herbology, perhaps with a new greenhouse dedicated to rare and dangerous plants for advanced students to study.
Harry himself could be the main character. Since it's a Harry Potter fanfiction, it's likely that he plays a central role. Maybe he's trying to impress Ginny by bringing her flowers in a very magical and unexpected way.
I recommend the following novels to you:
Professor Hogwarts in Marvel: The protagonist is Gilderoy Lockhart, a wizard who can travel back and forth between Harry Potter and Marvel. Watch how he is active in both worlds.
"The Wizard Who Arrives in the Beautiful World": The main character is Abel. He brings the magic and knowledge he learned from the Harry Potter world to the Marvel World, which is countless times more dangerous than the Harry Potter world.
"From Kama Taji to Hogwarts": The protagonist is Sven. The genius mage of Kama Taji came to the world of Harry Potter and combined the spells of Kama Taji with the spells of the world of Harry Potter to create a new magic.
I hope you like my recommendation. Muah ~