You can try fanfiction websites like Archive of Our Own (AO3). It has a vast collection of fanfictions from different fandoms, and there's a good chance you'll find some related to Airachnid and Jack there.
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 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.
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 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.
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.
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.
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 great fanfiction I read had Arcee and Jack teaming up to outwit Airachnid. The story starts with Airachnid targeting Jack for some unknown reason. Arcee, being Jack's protector, steps in. The fanfiction then takes us on a journey of how they discover Airachnid's motives and try to stop her. It has some really cool battle scenes and emotional moments between Arcee and Jack.
Fanfics often take existing characters and put them in new situations. In the case of 'airachnid x jack fanfic', it could be set in an alternate universe where the normal rules of the original story don't apply. It could be a story full of action, with Airachnid and Jack fighting side by side against a common enemy. Or it could be a more character - driven piece, exploring their feelings, motives, and how they view the world around them.