One great 'carla connor fanfic' could be 'Carla's New Beginning'. It shows Carla in a new light, exploring her character growth after a major life event. It has engaging dialogue and really gets into her psyche.
You can try fanfiction.net. It has a wide range of fanfics including those about Carla Connor. Just use the search bar and type in 'Carla Connor' and you'll get a list of relevant stories.
Well, it really depends on different fan preferences. Some might like a Connor who is a powerful fighter with unique semblance skills. For example, a Connor who can manipulate time in some way, like slowing it down during battles. This kind of characterization often makes for exciting action scenes in the RWBY fanfiction world.
It could be an older, more experienced android. One that has been through a lot and has a deep understanding of both the android world and human society.
Hank would be a very likely character. He already has a complex relationship with Connor in the original story. If Connor starts to become deviant, Hank's reaction could be very interesting. He might be suspicious at first, but then as he sees the positive changes in Connor, he could support him.
A new villain could be introduced in the fanfic who is specifically out to get Connor Kenway. This villain might have a personal grudge against Connor or be on a mission to eliminate all the Assassins, and Connor becomes their prime target and ultimately falls to their schemes.