I like when the rule elements tell us about something more interesting about the character than just boring numbers and mechanics. When I started to plan my current D&D setting, I went through every class and subclass, decided which to include (which was most) and made sure I had a place in the setting for them. I know where eldritch knights and arcane tricksters originate, I have animistic cultures for druids, rangers and totem barbarians, I have ancestor worshipping culture for ancestral guardian barbarians and so forth. Then these things are not just mechanics, they will actually help to anchor the character into the setting, make them feel like part of it.