Fanfiction often takes liberties with the relationship between Izuku and Toga. It could also involve them teaming up against a common enemy. This would require them to put aside their differences and work together. Writers can use this to create exciting action - packed scenes and explore how they would communicate and cooperate in such a situation.
Toga is still alive in the manga. The creators haven't decided to end her character's journey with death. Usually, such major plot points are well-signaled and haven't happened for Toga so far.
The relationship could develop slowly. Deku would likely admire Tsuyu from afar at the beginning. Then, they could be involved in group activities where they start to work together more closely. Deku would notice Tsuyu's kindness towards others and her determination. Tsuyu, in turn, would see Deku's growth and his unwavering spirit. As they get to know each other better, their relationship would progress from acquaintance to friendship and then to something more romantic.
Maybe Toga uses his powerful abilities. He could have some special powers like super strength or the ability to manipulate time or space. With these, he might be able to stop Kikyo from harming Kagome.
In fanfiction, their relationship could develop through a common enemy. Suppose a new, powerful villain emerges who threatens both Toga's goals as a villain (in her own twisted way) and Izuku's mission to protect. They might be forced to team up against this foe. During their time together, they would learn about each other's strengths and weaknesses, and this could lead to a deeper understanding and a change in their relationship.
Their relationship could start with mutual self - interest. They both recognize the power and chaos the other can bring. For example, Toga's ability to transform could help Villain Deku infiltrate places, and he can offer her protection in return.