One common theme is rivalry. Bakugo's aggressive and competitive nature against Todoroki's more reserved yet powerful self makes for an interesting dynamic. Another theme is friendship development, as they often start off at odds but then find common ground.
No way. The manga portrays their connection through mutual respect and competition in their journey to become great heroes. A kiss is not part of that narrative at all.
In some fanfictions, Bakugo can become more empathetic. For example, if he experiences a situation where he realizes the pain he's caused Deku over the years. His aggressive nature might soften as he starts to understand Deku better. Deku, on the other hand, could become more assertive. Instead of always being the timid one, he might stand up to Bakugo more firmly in fanfiction scenarios.
Todoroki can start by simply being there for Deku. Just sitting with him in silence when Deku doesn't feel like talking. It shows that he's not alone. Also, Todoroki can share his own struggles. Since he has his own complex family situation, he can relate to feeling down sometimes. This can make Deku feel that his problems are not unique and that it's okay to not be okay.
A common theme is rivalry - turned - friendship. Since they start off as competitors, many fanfics explore how they overcome their initial animosity. For example, in some fanfics, they learn to rely on each other in battles.
One common theme is friendship. Despite their initial rivalry, fanfics often explore how Bakugo and Deku form a deep bond. Another theme is growth. They both grow as heroes and as people in these stories. For example, Bakugo learning to control his temper and Deku getting stronger.