There is a sliding scale between the "Railroad" and the "Sandbox" when it comes to RPG campaigns, and between pre-planned and on-the-fly content.
Railroads are pre-set linear story paths that the players can break by deviating from them; this is the main criticism that tends to be levelled at traditional published adventures. The storyteller in me wants a beginning, a middle, and an end for my campaign "season" just like a TV season, but I don't want to railroad the players. And I don't want to prep it all up-front!
Sandboxes are environments that let the plots be driven by the players; the GM leaves hooks and clues for them to find but the story follows the players' choices. The storyteller in me loves this, I love to build in player stories, but I don't want to prep a load of stuff that won't get used and I don't want to wing too much
But these aren't the only structure options, these are just the options we see in modules because they are the options
that are easy to publish. I'm looking for a low-prep option 3.