In many ways, Tony Stark's actions in the movies themselves prove he has a heart. For example, his decision to sacrifice himself in 'Avengers: Endgame' was a selfless act that showed his deep love for the universe and its inhabitants. In fanfiction, this can be explored even further. Writers might depict his internal thoughts and emotions more deeply, showing how his experiences, like his time in captivity in the first Iron Man movie, shaped his sense of compassion. They could also show his relationships, such as with Pepper Potts and Rhodey, and how his love for them influenced his actions and decisions. Fanfiction often delves into the unspoken aspects of a character, and for Tony Stark, it can be used to highlight the very essence of his heart.
Well, in fanfiction, one key instance could be when Tony is faced with a moral dilemma. For example, he might have to choose between saving a large number of civilians or getting revenge on a villain who wronged him. If he chooses the former, it's proof of his heart. In the official MCU, he often made such selfless choices, and fanfiction can echo and expand on those moments.
Love is also a big theme. It could be romantic love, like his relationship with Pepper Potts showing a softer side of him that proves he has a heart. Or it could be platonic love, such as his deep friendships with the Avengers and how he cares for them in the fanfictions.
Pepper Potts could be a big help. She knows Tony so well, and she could try to jog his memory by showing him things from their past together, like pictures or mementos. Rhodey could also be there for him. He could tell Tony stories about their military days and how they became friends, which might trigger some memories.
One common character paired with him is Bruce Banner. They could be gaming buddies, with Bruce using his scientific knowledge to help Tony improve his games or vice versa. Another is Pepper Potts. She might be shown as someone who tries to get Tony to take a break from his intense gaming sessions.
You can try fanfiction platforms like Archive of Our Own (AO3) or FanFiction.net. Just search for 'Proof that Tony Stark has a heart' on their search bars, and you should be able to find relevant fanfictions.
A fanfiction with Tony Stark having a heart attack during Civil War can open up numerous storylines. It could show the reactions of other characters like Pepper Potts, who would be extremely worried. His fellow Avengers might put aside their differences temporarily to ensure his survival. Also, it could lead to a deeper exploration of Tony's health issues that were perhaps overlooked before the heart attack, and how it affects his future decisions in the ongoing conflict.