A theme could be about the transformation of their relationship. The pregnancy might bring them closer together or create new tensions. Another aspect could be how they prepare for the new life, considering their different natures. Maybe Jack has to adapt to Airachnid's more alien ways of dealing with pregnancy, or Airachnid has to understand Jack's human concerns.
One possible theme could be the challenges they face as a couple with an unexpected pregnancy. It might involve how they deal with the reactions of others in their respective worlds.
I'm not sure specifically about Jack and Airachnid pregnant fanfiction. You could try searching on fanfiction websites like Archive of Our Own. They have a vast collection of fanfics, and you might be able to find something related to that pairing.
One common theme could be the power dynamic between them. Since Airachnid is a rather menacing character and Jack is a human, exploring how they interact in terms of power is often seen. For example, how Jack might overcome Airachnid's threats.
One common theme could be the 'forbidden love' aspect, as they are from different sides (Airachnid being a Decepticon and Jack being a human). Another might be the idea of redemption for Airachnid through her relationship with Jack.
One common theme is the battle between good and evil, with Arcee and Jack representing the good side and Airachnid as the villain. Another theme is friendship, like the strong bond between Arcee and Jack that is put to the test by Airachnid's actions.
A more complex plotline could be a story of redemption for Airachnid. Jack, with his kind heart, sees something in her that makes him believe she can change. He tries to convince her to abandon her evil ways. This would involve a lot of character development for Airachnid as she struggles with her nature and the possibility of a new path, with Jack as her guide.
Another theme is the exploration of an unlikely friendship or more. Given that they are on opposite sides in the original series, fanfiction often delves into how they could bridge that gap. It could involve them having to rely on each other in a dangerous situation in a way that changes their relationship dynamics.
Another theme could be the exploration of the impossible becoming possible in the realm of fanfiction. It would likely be a very creative take on the characters of Jack Frost and Hiccup, with the added element of pregnancy bringing in elements of family, responsibility, and new beginnings in a completely out - of - the - ordinary way. Since this is such an unusual concept, the writer would have a lot of room to play with different themes and ideas.
One popular story could be where Airachnid's love for Jack is a forbidden love due to their different factions. They have to secretly meet and communicate, adding a lot of suspense to the plot.
One possible plotline could be the challenges they face in their relationship due to the unexpected pregnancy. Maybe they have to deal with the reactions of their friends and family.
Yes, there are some. You can start by looking on popular fanfiction websites like Archive of Our Own. Just search for 'Airachnid and Jack' and you might find some interesting stories written by fans.