It could symbolize freedom. In the very restricted society of 'The Giver', their kiss might be the first step towards breaking free from the imposed emotional numbness. It represents a new beginning, a way to explore feelings that are usually suppressed.
Well, it could show a moment of rebellion. In 'The Giver', the society is so controlled. A kiss between Jonas and Fiona might be a sign of their awakening to true emotions. It could be a very tender moment, like a small act of defiance against the strict rules of their world.
It would likely become more intense and protective. Jonas would be extremely protective of Fiona and the unborn child, given his experiences and the knowledge he has. He would be determined to keep them safe from any harm that the community might pose.
One possible plot could be that they face opposition from the community. Since the community in 'The Giver' is so strictly controlled, their pregnancy might be seen as a deviation from the norm. They could be forced to go into hiding to protect their unborn child from the authorities who might want to take it away or 'correct' the situation.
The main characters are clearly Jonas and Fiona. They are likely the focus of the story as the title implies that they are the ones who are pregnant (in the context of the fanfiction).
It could also be about the reactions of their friends and family. Some might be supportive, while others could be against it due to the norms of their world. The story could then focus on how Jonas and Fiona deal with the different reactions and still stay true to their decision to have the baby.