Well, if we consider their personalities, the Grandmaster is quite eccentric and Loki is complex. In the fanfic, the pregnancy could be a plot device to explore their relationship further. It could lead to them having to work together in ways they never expected, like the Grandmaster having to protect Loki during the pregnancy or Loki using his powers to ensure the safety of the unborn. There could be a lot of comical and tense moments as they navigate this new situation.
One possibility is that it could be a very strange and unique story. Maybe it would involve some sort of magical or other - worldly pregnancy. Loki is known for his trickery, so perhaps he got involved in a situation with the Grandmaster that led to this unexpected pregnancy in a very convoluted way.