@Cawalope urgh, I have this problem too... Some things that I'd remember from a few shows would be a love story ofc, something about a hobby (like a theater, something about music, bands), school projects (for relationships, maybe a group of people that don't like each other need to do a presentation, then end up learning a lot from each other), regular vacations/trips (could be helpful for relationships and worldbuilding), supporting a side character that is in a bad relationship with somebody or needs help (like bullying, self love, coming out) or perhaps just some kind of training, whether that be magic, fighting or learning that don't have a strong connection to the plot but could be helpful in the future.
I honestly don't know, I'm also fairly bad at coming up with good sub plots, we can just try to make notes during a show to get ideas...