Think of a concept - any concept - and roll with it. A single sentence can become a grandois story. Examples: Take a magic ring to a volcano. Go shoot an asshole with a diamond horse in the face. Go kill a dragon by yelling at it.
et cetera, et cetera.
Now you have your overall concept. Your beginning and end. now you focus on the parts in between.
If you are writing a book, you write one event after the other. cause, effect, cause, effect.
Don't do that.
With tabletop games, there's an unstable element within the players. A human element that's going to be incredibly unpredictable.
A player may decide that **** your ring I want to build a farm. **** your diamond pony I'm gonna go run a bank. et cetera.
You can not account for the effects for causes that haven't and might not happen yet. Instead of trying, the correct formulae is this
Start, and then some things happened, event, and then some things happened, event, and then some things happened, end.
Think of it this way: in Doctor Who, you have major "time-locked" events that have to happen. those are your planned events. You also have tons of events that don't have to happen. at all. those are your "and then some things happened."
Side quests should be in the "and then some things happened." and can be used to lead the players towards events.