His character might become more confident, knowing he has the most powerful items in the universe at his disposal. He could be more decisive in his actions and less hesitant about taking risks.
In fanfiction, once Tony Stark starts using the Infinity Stones, his character development can be complex. On one hand, he could grow into a more powerful and benevolent figure. He might use the stones to improve the lives of everyone in the universe, which would make him more of a hero than ever. But on the other hand, the power of the stones could go to his head. He could become arrogant and start believing that he is the only one who can make the right decisions. This could lead to conflicts with his friends and allies, as they may see him changing for the worse. Also, the experience of using the stones could make him more aware of the fragility of life and the importance of balance in the universe, which could lead him to be more cautious in his future actions.
When Tony Stark uses the Infinity Stones in fanfiction, his character could develop in different ways. He might become more selfless as he realizes the great responsibility that comes with such power. He could also become more isolated as others may fear his new - found power or he may feel that he is on a different level than everyone else. He may start to question his own morality as he has the power to do almost anything, and deciding what is right and wrong becomes more difficult.