Definitely not. Such content objectifies the characters in an improper way. These characters are part of a story that is meant to be enjoyed in a more positive and respectful way, not through creating sexual scenarios.
It's not okay at all. Fanfiction should be based on positive and ethical interpretations. Thanos is a villain, and Gamora has her own story arc that doesn't involve this kind of inappropriate relationship. Moreover, this kind of content can be disturbing to a lot of people who love the original franchise. It's better to focus on creating fan works that explore the characters' growth, their battles, and their relationships in a proper and respectful way.
No. Writing sexual fanfic about characters is inappropriate as it violates the respect and integrity of the original characters and can make many fans uncomfortable. It also goes against the general ethical and moral standards within the fan community.
There are several negative impacts. Firstly, it is morally wrong as it involves improper and unethical content especially considering Gamora's character. Secondly, it can have a negative influence on the perception of the characters in the Marvel universe. People may start to view Thanos or Gamora in a wrong and inappropriate light. Thirdly, it can lead to a division in the fan community, with some fans being disgusted by such content and others maybe wrongly thinking it's okay.
Their relationship would likely deepen. The impending arrival of a baby would bring them closer together on an emotional level. They'd have to communicate more about their hopes and fears for their family. For instance, they might have long talks about what kind of values they want to instill in their child.
Another plot could involve them preparing for the arrival of the baby. They could be seen shopping for baby supplies in different alien markets. Quill would be clueless about most things, while Gamora has a more practical approach, leading to some comical situations.