In the Marvel Cinematic Universe, one of the abilities of the Scarlet Witch was to control the mind. However, in Avengers 3: Infinite War, the Scarlet Witch did not use the ability to control her mind because she did not think it was necessary.
In the movie, the Scarlet Witch and Thanos had a fierce battle. In this battle, the Scarlet Witch realized that she needed to use the power of mind control to defeat Thanos. However, she found that this power would make her weak and might even cause her to lose control of herself. Therefore, she decided not to use this power but to use other skills to defeat Thanos.
In addition, the Scarlet Witch also believed that she had the ability to use other skills in battle without relying on Mind Control. For example, she used magic to create an illusion to help her in battle. Therefore, she chose not to use the Mind Control ability and instead used other skills according to the specific situation.
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.