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.
I think Christmas piano cartoons that have a heartwarming story behind the music tend to be well-liked. Maybe like a group of friends coming together to play piano for a charity event during Christmas. Also, ones with colorful and magical settings can attract a lot of viewers.
You could have them constantly trying and failing at diets or exercise routines in a hilarious way. Or maybe they have a love for food that leads to funny situations. Another option is to have other characters' reactions to them be funny, like being overly shocked or scared of their size.