The popularity of Thor x Loki fanfiction lies in their complex relationship. They are brothers but also have a lot of conflict. Fans like to explore how they can reconcile or what makes their relationship so strained.
It could be Captain America. He has a strong sense of justice and kindness. So, if he found a dog, he would surely take it under his wing and protect it. Maybe he'd even train the dog to be a mascot for the Avengers.
In some fanfictions, the 7th Avenger could be a time - traveler from the future. This person has come back to prevent a major disaster that the Avengers couldn't handle without their help. They know all about the future events and use this knowledge to guide the team, but also have to be careful not to change the timeline too much.
Percy could also be a big fan of Hulk. Just like Percy has his own 'inner beast' that he sometimes has to control (his demigod powers), Hulk has his own anger issues. Percy might see a bit of himself in Hulk and also be in awe of Hulk's raw strength. He would probably be interested in seeing if there's a way to help Hulk better control his rage, using his own experiences of self - control.
There have been many suspects over the years. One theory pointed to a person with a connection to the seedy underbelly of Hollywood at that time. This person might have been involved in illegal activities and had a disturbed mind. But without conclusive evidence, it's all speculation. The case is complex as there were many people who could have had motives, including those who knew the victim Elizabeth Short on a personal or professional level.
Yes, perhaps in this fanfiction Bella could be a new Avenger. Fanfictions often introduce new characters or give existing characters new roles within the established Avengers universe.