Ok there’s the standard idea for starting a quest. An old man says that the wicked necromancer is kidnapping the locals to use in his experiments. Your contact in cyberpunk tells you that Gideon, who runs the local pawn shop, sells info on his clients to the corps. Local preacher says that there’s a powerful manitou that lurks in the abandoned mine shaft. These ways of giving quests are straightforward. There’s a problem and your players can go to resolve it, usually with a final result in mind. But how about a quest with no “final result.” Just a vague question that sits in the back of players minds.
Enter the “Interesting Mcguffin.” This kind of item can usually be introduced underhanded, almost as an afterthought. For instance, your posse from Deadlands just killed a Harrowed gunslinger. You’re rifling through his pockets to see if you can get any identification of who he was or how he became harrowed. In the breast pocket of his shirt you find a bullet, jet black. There’s no makers mark on it. And your huckster gets a bad feeling when he’s holding it. So what is the bullet, why was it made, what was its purpose, and how did this Harrowed get a hold of it? That’s your Interesting Mcguffin.
You need to make the Mcguffin puzzling enough so that between the “linear missions” your players want to spend time unravel the mysteries of the Mcguffin. It can be anything depending on your setting. A broken piece of an ancient tablet with weird writing on it. An encrypted piece of data that sticks behind in your datalock. A flower that never wilts but changes colour for some reason. You as the GM know all about it and what it does, but your players don’t.
There’s an upside and a downside to these kind of items. Downside is that some players just chuck it in with the rest of the loot and forget about it. Or they may, inadvertently, find a way to get all mysteries resolved with no issues. Don’t worry if that happens. Just run with it and don’t try to force things with your players. The trick as a GM is not to be “invested.” Just make sure that you remember what your players did with it, even if they forgot. Like when your players are stopped by security and ordered to turn our their pockets. Now they have to think on their feet and explain the “interesting Mcguffin” to their captors.
The up side? It gets your characters thinking. Not just Out of character, but in character as well. Your science type may want to analyze it. The greedy thief may want to try and pawn it (with sometimes funny results). The Lawman may want to get it back to its owner. Your mystic may want to unravel its deep secrets.
Nice thing about this kind of item is that it can eventually turn into a quest in and of itself. They may abandon their current quest to figure it out.
Just be prepared to think on your feet. The Mcguffin’s inherent flexibility can have the characters approach it out of left field sometimes.