There might be a friend of Harry's, like Hermione. She could be the one who tries to find a way around Harry having to pay death. She might research in the library, looking for ancient spells or forgotten lore that could help Harry avoid this dangerous payment while still achieving time travel. Also, there could be a villain who wants to stop Harry from paying death and time traveling because they fear what Harry will do once he has that power.
Perhaps there is a magical creature that is the guardian of the time - travel portal. This creature, let's say a phoenix - like being, has the power to either allow or deny Harry's access to time travel. It might be an ally or an obstacle depending on whether Harry can meet the condition of paying death. And there could be a mentor figure from Harry's past, like Dumbledore's ghost, who appears to give Harry advice on how to deal with paying death for time travel.
Hermione could be important. She might help Harry in his journey as a mage, using her intelligence to research and understand new magical concepts that Harry discovers. Ron could also be there, providing his loyalty and friendship, perhaps even joining Harry in some of his mage - related adventures. And then there could be a new character, a powerful mage who becomes Harry's mentor and teaches him the deeper secrets of magic.
Besides Harry, death is the central character in this concept. But there could be other magical creatures or beings involved. Maybe thestrals, since they are associated with death in the original Harry Potter lore. They could play a role in facilitating the friendship between Harry and death. And of course, Harry's other friends from Hogwarts, like Neville and Ginny, could also be part of the story, either being curious about or affected by Harry's friendship with death.
Hermione could be a main character. She might be the one who first notices the strange changes in Harry after he becomes friends with Death. She could try to research about this unusual friendship in the library, using her vast knowledge to understand the implications. Ron could also be important. He might be initially skeptical about Harry's new 'friend' but then come to accept it as he sees the positive effects it has on Harry and the help it provides in their fight against dark forces.
There could be a local wizard who becomes Harry's mentor in a way. Since Harry is on his own, he might seek guidance from someone outside of his usual circle, like an old hermit - like wizard who has a lot of knowledge about the more hidden parts of the wizarding world.
In many of these fanfictions, the Marauders (James, Sirius, Remus, and Peter) are quite popular. They were friends of Harry's parents and their presence in a time - travel story can bring a sense of nostalgia and also add a lot of action and adventure. Bellatrix Lestrange can also be a prominent character on Voldemort's side. Her fanatical loyalty to Voldemort and her dark magic skills can make for some intense and exciting scenes, especially when she clashes with the heroes in different time periods.
Maybe the 'paying death' is a ruse. Death is actually an entity that wants to test Harry's worthiness to time travel. So, Harry is made to go through a series of trials where he thinks he's paying death but in reality, he's just proving his courage, intelligence and loyalty, which are the real keys to time travel in this fanfiction.
Obviously, Harry Potter himself is a crucial character. He is the one 'chosen by death' and the whole story likely revolves around his experiences and growth. Hermione Granger would also be important. Her intelligence and research skills would be valuable in helping Harry understand his new situation. She might be the one to dig up ancient texts and lore about being chosen by death. Ron Weasley too, as Harry's best friend, would be there to support Harry, even if he doesn't fully understand what's going on at first.
There could be original characters created specifically for the fanfiction. For example, a wizard who has some knowledge about the origin of the Omnitrix and tries to guide Harry. This character could be an old hermit - like wizard who has been studying interdimensional objects for years. Also, there might be a villain who is after the Omnitrix, someone who wants to use its power for evil purposes and constantly hunts Harry down.
Hermione could be important. She might be the one who encourages Harry to drop his mask, or she could be the first to notice the change when he does. Another could be Ron, as Harry's best friend, his reaction to Harry dropping his mask would be significant.
Hermione would probably be involved. She could help Harry research about the island, like its history and any magical properties it might have.