One popular straykids fanfiction is the 'Soulmate AU' ones. These often explore unique and creative ways the members could be connected as soulmates in different universes. Another is the 'College AU' fanfictions which place the members in a college setting, dealing with typical college experiences like exams, friendships, and of course, romance. There are also those that are based on fantasy worlds where the members have special powers.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of straykids fanfiction and you can search by tags such as the member's name, the type of AU (Alternate Universe) you're interested in, or the genre like romance or angst. Another option is Wattpad, which also has a decent number of straykids fanfics. You can also follow straykids fanfiction blogs on Tumblr, as they often recommend good stories.
Max Schreiber is quite interesting. He's a wizard with his own motives and powers that often play into the adventures of Gotrek and Felix. His magical abilities add an extra layer of complexity to the story.
Yes, Felix the Cat holds the title of being among the first cartoons. It had unique features and charm that contributed to the development of the cartoon industry.
I'm not sure specifically what 'felix fanfic' is about without more context. It could be a fanfic related to a character named Felix. Maybe it's about Felix's adventures, relationships, or his role in a particular fictional world.