Well, it could start with Jon and Daenerys arriving in Pentos. Maybe they are in hiding or on a diplomatic mission. Then Daenerys discovers she is pregnant. This could lead to a whole series of events. They might have to deal with local politics in Pentos as they try to protect their unborn child. Jon could be worried about the safety of his family in a foreign land, and they might face threats from various factions who either want to use the baby for their own gain or are against their union.
In a 'jon daenerys pregnant pentos fanfic', the plot could be that they are in Pentos to seek refuge. Daenerys being pregnant adds an extra layer of complexity. They may need to find allies in Pentos to help them during the pregnancy and the birth. There could be a conflict between Jon's northern roots and Daenerys' more southern and exotic upbringing. As the pregnancy progresses, they might face challenges related to health, as well as political intrigue. And the story could end with the birth of their child and a new chapter in their lives, either in Pentos or with plans to return to their homeland.