Their relationship could grow stronger as they face the challenges of pregnancy together. Franky would become more protective, using his mechanical skills to ensure Robin's comfort. For example, he could build a special chair for her. Robin would rely on Franky more, sharing her fears and hopes for their future family.
Their relationship could start with distrust. Aokiji being a former Marine and Robin being a wanted archaeologist. But as they interact more, they might find common ground in their search for truth. Maybe they start to share stories about their pasts, which slowly builds a bond between them.
Their relationship could develop in a more academic way. Since Robin loves knowledge, she might start teaching Luffy about the history of the world. Luffy, being curious, would listen intently and ask simple but profound questions. This would make Robin see Luffy in a new light, not just as a captain but as someone with a thirst for knowledge as well.
It might change in terms of their communication. They would have to discuss more about their future and the future of the baby. For example, they would need to decide where to raise the child, whether in the forest with the Merry Men or find a more settled place. Their relationship would also become more tender as they anticipate the arrival of their baby.
Robin could use his intelligence to analyze Deathstroke's lair and find a weakness in its security system. He might notice a pattern in the guards' patrols or a flaw in the locking mechanisms. Then, he could use his acrobatic skills to create a distraction and slip away unnoticed.
Their pregnancy could bring them closer. They would have to rely on each other more, sharing responsibilities like doctor visits and preparing for the baby.
Often, it starts with their mutual respect. Luffy respects Robin's intelligence and strength, and Robin respects Luffy's leadership and unwavering spirit. Their relationship then grows as they share adventures and overcome challenges together. For example, in many fanfictions, during a battle where Robin is in danger, Luffy's fierce determination to save her deepens their bond.
Their relationship could start off as a bit of a mentor - mentee thing. Catwoman would be the experienced one showing Robin the ropes in her way of doing things. As time goes on, they could become more like partners in crime - fighting, with a deep bond of mutual respect.
Archive of Our Own (AO3) is a great place to start. It has a large collection of fanfics from various fandoms, including One Piece. You can simply search for 'Franky x Robin' in the search bar and it'll show you relevant stories.
She would likely become more open. At first, young Robin might be reserved due to her past experiences. But as she spends time with the Straw Hats, she'll start to share more about herself. For example, she could gradually open up about her family and her dreams for the future. Her confidence would also grow. The Straw Hats' acceptance and support would make her believe in herself more. She might become bolder in using her skills, like when exploring dangerous ruins or facing powerful enemies.
The relationship could also develop because of a misunderstanding. Maybe Blackfire is trying to help the Teen Titans in her own way, but Robin misinterprets her actions as being sneaky. This causes a rift between them at first. But as they work through the misunderstanding, they learn more about each other. Blackfire explains her motives, and Robin realizes he was wrong. This new understanding could lead to a closer bond, with Robin seeing Blackfire in a new light and starting to develop feelings for her.