Plot Items with no plot: Making it seem like you have it all planned out
Problem: we want to run a game, but we're lazy and don't have a full story totally built.
Problem 2: we don’t want to plan too far out and force players into a certain story, so it has to be flexible to avoid railroading anyway.
Problem 3: despite this, we are kinda expected to have plans for stories that span for years and reveals that come after dozens of sessions of buildup.
Answer: give yourself the tools to improvise by providing them with plot items that will recur throughout the campaign and play a major role in all its major stories while giving them mysteries and side quests before you even know what the item is for.
Let's unpack that a little.
Our goal is to give them an item (does not have to be, but for the purposes of this post let's go with it, items are easy to hand out, expected loot in DnD) that will give the players a mystery and a clue. We do not have to know what that is yet, but just the hook is enough to convince them that the mystery is there, and that it is their job to figure it out.
The ideal is something that can be given to characters of any level, that provides multiple plot points, and builds up over time to an eventual resolution. We have no idea what those will be yet, but by giving the item, the players think that we do. Their paranoid conspiracies thoughts on the matter can be fed right back into the campaign to show you what they want to see. And as you have a better grasp of the campaign, you can tie the story together using the item.
The trinket table in the PHB is great for this sort of fuel, almost any of them can be turned into a great plot hook. But here’s some other examples.
Example 1: The single sending stone.
Players find a single sending stone in the ruins of some random dungeon. Apparently, it still seems to work. Somebody curious asks who is on the other side. There is a long pause. "Who are you," asks a deep, mysterious voice, "how did you get this sending stone?" Whatever the players say, they receive no answer immediately.
I have seen this a few times, because it is a great hook. You can drop it anywhere and explain later. The other side is a total mystery, they could be the BBEG, a lost parent, the king, a mad mage, who knows? See who takes an interest in the stone and figure it out from there. All you need to make this work is a voice.
Example 2: The strange key chunk.
Better that this isn't obvious as a key, but perhaps an odd gem, or a set of statues, or part of a set of jewelry. When they find it, they may think nothing of it, except that it is clearly part of an incomplete set. They now have a quest to find all the remaining pieces, as well as discover why they were split up, and what they open when they are finally put together.
Something I've used myself to great effect, you can make this key a long adventure to find not only all the pieces, but also the door. What is the door to, where is it, what is behind it? Who knows, but I am sure you can find a place for it in the story as it develops.
Example 3: A Letter
A little like the sending stone, but even more mysterious, would be a menacing letter without a name either to or from, or maybe just a single initial. You can put just about anything on the letter, but the more generic, the easier it is to use. “You’ve held your part of the bargain so far, find the payment in the agreed location as well as your next instructions.” Similarly, a map with no specific reference points except for a few places circled gets the same point across by producing a mystery that you can deal with later.
TL:DR; want to hook your players on a long campaign even though you barely have the first session planned? Give them a part of a key. Figure out what it goes to later, they'll think it was all planned.