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.
It depends on your preferences. The anime brings the story to life with dynamic visuals and sound, but the manga offers more detailed panels and a unique reading experience.
One of the best SNK fanfics could be those that focus on the backstories of the characters. For example, a fanfic that delves deep into Levi's past before he joined the Survey Corps. It might explore his life in the underground and how he developed his combat skills. Another great one could be about Eren's internal struggle in a more detailed and alternative way than in the original story. There are also fanfics that explore different relationships between the characters, like a friendship - centric story between Armin and Mikasa that shows their bond growing in unique circumstances.
One characteristic could be its unique art style. SNK is known for its distinct visual aesthetics, which are likely to be carried over into the visual novel. Another might be its storytelling. It could potentially have complex and engaging plots, perhaps with elements of mystery or action.
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.