Well, characters like Dr. Strange might be a bit critical. He'd notice the inaccuracies in the use of magic in some fanfictions. However, he might also appreciate the efforts of fans to incorporate his character into different and exciting scenarios. The more down - to - earth characters like Falcon might be more accepting and just enjoy seeing their world expanded in new and different ways through the fanfiction.
Characters' responses could be mixed. Scarlet Witch could be intrigued by how fans explore her powers and her emotional state in fanfiction. She might see it as a way to understand how she's perceived by the public. War Machine, on the other hand, might be a bit dismissive at first, but as he reads more, he could start to see the charm in the fan - created stories about the Avengers.
Some characters would likely be entertained. For instance, Loki might enjoy seeing the various ways fans write him as a more complex anti - hero. He'd probably find the creativity in the fanfiction quite appealing.