One common theme is empowerment. Writers often depict the female Ratchet as a strong, capable character who can take on any challenge in the universe. She might be shown to be even more resourceful than her male counterpart in some stories.
Another option is to join Ratchet and Clank fan forums or groups. People there often share and discuss their favorite fanfictions. They might be able to point you directly to some great stories with a female Ratchet. For example, Reddit has some active Ratchet and Clank communities where fans exchange such information.
Ratchet could introduce her to his friends or colleagues who can also offer support. Having a wider support network can be very helpful for someone with cancer. He can also engage in activities with her that she enjoys, like reading books together or watching movies, to take her mind off her illness.
One reason for the popularity of Optimus x Ratchet fanfic is the opportunity to explore their relationship in different scenarios. Maybe in a fanfic, they get stranded on a strange planet and have to rely solely on each other. This kind of situation can bring out new aspects of their bond that we don't see in the original series.
Well, 'construction' in this context might refer to building something within the fictional world of the fanfic, like structures or relationships. 'Ratchet' could imply a bit of a rough or unrefined aspect. Fanfic, of course, is fan - created fiction. So it could be about a story where there are elements of building or creating within a somewhat rough or edgy setting in the world of a particular fandom.