I'm guessing sidequest is just quests that don't always start in your opening hand?
going by how they worked in the tests they did on ungoro I think that's how they work. I wanted to know if the side quests can be played alongside a quest, but I'm not sure yet
EDIT: this is not exactly a new idea, they wanted to try this out on ungoro, but that expansion ended up having too much and they left it unadded