Well, one possibility is that Bakugou is initially frustrated with the hero society in the fanfic. Maybe he feels overlooked or misunderstood. The League of Villains could approach him, promising a place where he can use his power more freely without the restrictions of the hero rules. And he might be tempted by their offer, leading to his joining.
One interesting storyline could be exploring the backstories of the League of Villains members in more depth. For example, delving into how Tomura Shigaraki became the way he is. Maybe he had a tragic past that led him to embrace villainy. Another could be a story where the League of Villains teams up with an unexpected group for a common goal, like a group of anti - hero vigilantes. It would create a lot of moral and ethical dilemmas for both sides.
His aggression might become more unhinged. Without the moral compass of the hero side, he could be more willing to use excessive force. He'd still be competitive, but now it would be against heroes instead of his fellow hero trainees.
Tomura Shigaraki is a very popular character to write about. His complex personality and his connection to All For One make him an interesting subject. His journey from a seemingly lost and angry young man to a powerful villain leader has a lot of potential for exploration. Dabi is also quite popular. His mysterious past and his relationship with the Todoroki family add depth to his character. Writers can play with the idea of his motives and what he really wants out of his villainous actions.
The main differences are in their expressions and some minor details of personality. In the manga, Deku might seem a bit more intense, while in the anime, he's sometimes presented a bit softer.
In 'deku meets future deku fanfiction', perhaps the future Deku warns the present one about upcoming threats. He could tell him about villains he'll face in the future and how to defeat them. This would give the present Deku an edge in his battles and also in his growth as a hero.
Well, obviously Deku in two forms - the younger and the older version are the main characters. The younger Deku is the one who is still growing and learning, while the older Deku has more experience.
As of now, Deku is very much alive in the manga. The plot hasn't reached a point where he meets such a fate. Usually, main characters like Deku have significant story arcs and aren't easily killed off.
Another possibility is that a new villain emerges as a result of the time - travel or whatever mechanism brings future Deku to the present. This villain could be from a future where Deku's actions (or inactions) have led to a more dangerous world, and he travels back in time to stop Deku from making those mistakes again. However, his methods are extreme and dangerous, making him the antagonist of the story.