Yes, in some fanfictions, Harry could be depicted as the last wizard. This might be part of a post - apocalyptic or very unique magical world setting where all other wizards have perished for various reasons, like a great war against dark forces that wiped out most of the wizarding population and Harry is the sole survivor.
I'm not sure specifically as there could be many different fanfics with that title. It might be about Harry Potter in his final battle as a wizard, perhaps with new and unexpected challenges, allies, or enemies.
One significance could be the exploration of extreme isolation. Harry would have no one to share his magical knowledge or experiences with in the wizarding sense. It could lead to a very different character development as he has to re - define his identity without a wizarding community.
Most likely Harry Potter would be a main character since his name is in the title. And probably his friends like Hermione and Ron would also be involved.
It could be about the last wizard in the Harry Potter world somehow getting involved with the Marvel universe. Maybe the wizard has to seek help from the Marvel heroes to save the wizarding world from a new and powerful dark force. Or perhaps the wizard brings a unique magic that the Marvel world has never seen before and causes a stir among the superheroes and villains alike.
If Karin had Kryptonian abilities, her confidence would likely soar. She could become more assertive in her actions. For instance, with super strength, she would no longer be as afraid in combat situations. Her character might also become more isolated at first as she tries to figure out her new powers on her own, not wanting to endanger her friends. But as she masters them, she would be more open to sharing and using them for the good of the group.
Naruto could use his chakra control to enhance his Kryptonian super speed. For instance, he could focus chakra on his legs while using his Kryptonian speed to move even faster and more stealthily. Also, his shadow clone jutsu could be combined with his Kryptonian powers. He could send clones with super strength or heat vision to attack enemies from different directions.
There is. You can often find them on various fanfiction websites. They usually put Harry in the DC universe as a Kryptonian. The stories might involve him teaming up with Superman or having his own adventures while dealing with his new identity and powers.
One idea for a Peter Parker and Kryptonian fanfiction could be that Peter discovers some Kryptonian technology while on a school trip to a museum. Maybe it's a small crystal that has some latent Kryptonian power. He starts to have strange dreams about Krypton and is drawn into an adventure where he has to team up with a young Kryptonian who has also landed on Earth. Together, they fight against a common enemy that is threatening both their worlds in a way.