In theory, no. Fanfiction is a creation of fans in our world, and the AOT characters exist within their own fictional universe. There's no mechanism in their world that would allow them to access fan-created stories about their future. They are bound by the rules and plotlines set by the original creators of Attack on Titan.
The ending of the AOT manga was quite controversial among fans. Some were satisfied, while others felt it was rushed or left many questions unanswered.
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.
One way is to check on popular fanfiction websites. Look for ones with high ratings and a lot of reviews. For example, Archive of Our Own (AO3) has a large collection of AOT fanfictions. You can search for the most popular ones there.
If AOT characters learned their future fanfiction, it could have a huge impact on their actions. For example, Eren might change his entire plan if he saw some fanfictions where his actions led to even more destruction than he intended. Characters like Mikasa might be really confused or even angry about some of the fan - created relationships she's involved in that go against her loyalty to Eren. And Levi, he might be appalled by some of the out - of - character or overly dramatic situations written about him in fanfiction. In general, it would disrupt the natural flow of their original story.
Eren is often a popular choice. Since he has such a central role in the original story, seeing him interact with the past or future in a time - travel scenario is really interesting. He could go back and try to change things before he made certain decisions that had far - reaching consequences.
Definitely not. AOT is a work of fiction that combines elements of fantasy, action, and drama. It's set in a made-up world with its own unique plot and characters.
The AOT manga has finished. It wrapped up all the major plotlines and provided a conclusion to the story that fans have been following for a long time.