A common theme is their bond. Since they are friends in the original story, fanfics often explore how deep that bond can be. It could be a story about loyalty, where they are always there for each other no matter what.
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.
The theme of loyalty also pops up often. They are shown to be extremely loyal to each other. In some fanfics, when one of them is in danger, the other will go to great lengths to protect them, no matter the cost, which really emphasizes the strength of their relationship.
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.
He might cry a lot. Deku is very emotional, and being cheated on would be a very painful experience for him. He could also get angry, which is rare for him, but this situation would be so unjust that he might not be able to hold it in. He'd probably start to question his self - worth too, thinking that there was something wrong with him that made his partner cheat.