Princess Bubblegum would be a great addition. She could offer her scientific knowledge to help Cake and Jake in their adventures. For example, if they encounter a strange chemical reaction or a mechanical problem, she could come up with a solution. Finn is also a good choice. His bravery and sword - fighting skills would be useful when they face dangerous situations. And Marceline the Vampire Queen could add an element of mystery and her vampire powers might come in handy at times.
People who like mystery and suspense would probably enjoy it. If the story has elements of mystery, like hidden secrets about Jake Ellis or some kind of overarching mystery to be solved, it would attract mystery lovers.
I think Jake Paul would be the more likely protagonist. He's the one from our world, and the fanfic could be about his adventure in the world of Thanos. He would have to adapt to the new environment, learn about the powers and rules there, and ultimately face Thanos. His journey of self - discovery and growth while dealing with Thanos would be the main focus of the story, making him the protagonist.
I haven't come across any reports stating that Jake Tapper is penning a novel. His focus seems to be on his journalistic endeavors, such as reporting the news, conducting interviews, and providing analysis. However, it's not impossible for him to venture into novel - writing in the future, but for now, there's no indication of it.