Maybe the pregnancy is actually a part of a bigger plan by an enemy of the mob boss. They use Edward and Bella's situation to get to the mob boss, for example, by trying to kidnap Bella when she's most vulnerable. The mob boss then has to decide how much he's willing to risk to protect them.
The mob boss might be initially suspicious. He could think it's a ploy by an enemy to gain his sympathy or get closer to him. So, he might keep a close eye on Edward and Bella at first.
One possible plot twist could be that the pregnancy is not natural. Maybe it was the result of some magical or supernatural interference. For example, a powerful witch cast a spell on them for some unknown reason.
One idea could be that Bella is a journalist trying to expose Edward's mob activities, but she ends up falling for him instead. Another could be Edward protecting Bella from rival mobsters when she gets caught in the crossfire of his underworld business. A third plot could be that Bella is a singer in a club owned by Edward's mob and they develop a relationship through her performances at his establishment.
One possible plot twist could be that the kidnapper is actually someone from their past who has a vendetta against them. Maybe it's a vampire who was wronged by the Cullens long ago and is now using their son as a pawn for revenge. This would lead to a tense situation where Edward and Bella have to face their family's history while trying to rescue their son.
A great plot twist could be that the pregnancy is not as straightforward as it seems. There could be some sort of magic or curse involved that changes the nature of the baby as it develops. For example, the baby could start to show signs of being a different species than expected. Also, what if there are other characters who want to claim the baby as their own for power or some other ulterior motive? This could lead to a lot of conflict and drama in the fanfiction.
One common theme is protection. Since they are in the mob world, Edward has to protect Bella and their unborn child from various threats.
Another plot twist could be that the father of the baby is not who they initially thought. This would create a huge amount of drama and relationship turmoil. It could also lead to a search for the real father and a battle against whoever was trying to deceive them about the pregnancy.
Another theme is the idea of forbidden love. Edward's mob activities are illegal and dangerous, and Bella getting involved with him goes against the norms. Their relationship is often a struggle between their feelings and the consequences of being together. In many of these fanfictions, there's also the theme of protection. Edward uses his power as a mob boss to protect Bella from various threats, whether they are from his enemies in the criminal underworld or other external forces. This protection aspect adds an extra layer of complexity to their relationship.
A really interesting plot twist could be that Bella's pregnancy progresses at an extremely fast rate, and the baby has powers that allow it to communicate with Bella and Edward telepathically even before it's born. This causes them to reevaluate their relationship and what they thought they knew about vampires and half - vampire children.
One possible outcome is that Edward will use his mob influence to protect Bella's niece from any threats. Since he's a mob boss, he has power and resources at his disposal. Maybe he'll give her a life of luxury and safety she never had before.