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.
Bruce Banner, being a man of science and someone who has struggled with his own inner turmoil (the Hulk), would approach the situation very gently. He'd likely want to run some tests to see if there are any chemical imbalances in your body that could be contributing to your feelings. He'd also talk to you about how he copes with his anger and how you can find healthier ways to deal with your emotions, like he does with his research and helping others.
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.