Another difference lies in the overall mood. Canon AOT has a rather dark, gritty, and often hopeless mood due to the constant threat of Titans. However, a friendly Titan fanfic would likely have a more positive, hopeful mood. It might explore themes of redemption for the Titans, or how their existence could be beneficial to the world of AOT. This is a complete departure from the doom - and - gloom feeling that often pervades the official AOT story.
It's fairly common. Fans often look for more content related to their favorite shows. Since 'Attack on Titan' has a large and passionate fan base, there are a lot of fanfics available. 'Aot' fans might watch (read) these fanfics to explore different scenarios that the official story didn't cover. For example, they might want to see how a certain character would react in a different situation or what would happen if a major plot event had a different outcome.
In some aspects, the ending of the AOT anime varies from the manga. For example, the character developments and plot resolutions might not be exactly the same. These differences have sparked various discussions among fans.
A 'historia visual novel' could be about various things. It might be a story set in a historical context, with characters and plots that unfold in a way typical of visual novels. It could focus on a particular era, like the Renaissance or the Victorian age, and tell a fictional story with elements of that time's culture, society, and events.
There could be various fanfictions. Maybe some that focus on alternative endings, like ones where certain characters survive who didn't in the original. For example, a fanfiction where Erwin survives and leads the Scouts in a different way.
Yes, there are quite a few. You can start by looking on fanfiction.net or Archive of Our Own. Some fanfictions might explore their relationship in an AU (alternate universe) setting, like a modern-day scenario where they meet and get married, while others could be more in line with the original story's themes and settings but with a different plot twist leading to their marriage.