#Edge
I'm a huge fan of coming up with outrageous scenarios for my players and their characters, but I always try to think of three game changers that the characters could use for a significant edge over their enemy.
It's kind of laughable, because I'm batting exactly zero when it comes to predicting how the players react and what they will use to get an edge.
The benefit of thinking about how the main antagonist's plan could fall apart is not putting things on a platter for the players, but being able to react appropriately when it happens. Because he or she isn't the hero and the default winner, the party is.
In the Avengers film, they nailed Thanos's reactions. He told the heroes that their knees would be weak when they failed. Sure enough, Tony went down when he lost. When Thanos failed, he too sat down, weak-kneed.
That's good for a movie with an necessary ending, but the party's adventures don't always end with the defeat of the bad guy. Very often my Big Bads are made completely irrelevant by the characters and the mechanism of how that works is often tied to whatever edge they had in the conflict.
In one campaign, I had the party endlessly antagonized by a ghostly voice that whispered, "Silver is your enemy." The paladin made a very good leap of logic and asked if the voice sounded like their antagonist. I totally mean it to be the voice of their enemy, but the paladin pointed out that those words were only said after whatever ominous threat was given. In his mind, it sounded like a retort. I had meant it to be a tag line, but could those words be spoken by someone other than their enemy?
Well, after hearing a rather well reasoned argument from the paladin, I decided that it could be someone else speaking. So who's voice was it and what was it talking about? The paladin surmised that his god didn't like his minion threatened, so his god was issuing a threat to their tormentor.
"Silver is your enemy," did actually refer to many traits the paladin used to define himself. A silver decked horse, a silver sword, a silver symbol. Since their antagonist was extraplanular, silver was an effective defense.
Originally, I had meant for this extraplanular enemy to have an immunity to silver, at the cost of having a weakness to iron making the party's obvious strengths a weakness. But, once I had the player's input, I dropped that idea. It was going to be a straight up slugfest between the party wielding silver and demonic forces harassing them.
The demon was supposed to punch through into this dimension, but I flipped that around. The characters would be going to the demon's dimension AND they would possess all of the nasty, dangerous attributes of a demon on the prime material plane while there, with the Silvered Paladin acting as a locus of the power.
The party used that edge to the fullest, dishing out horrific damage on the forces of evil. But then they lost the fight and were dispelled back to the Prime Material Plane, forbidden from entering that dark realm for 999 years. They were startled to be back home, whole and healthy while the demon was horribly weakened and unable to strike at them directly.
That turned the whole adventure into a detective story, where the party had to figure out who the demon was using to continue it's attack. They managed to neutralize the demon at the cost of all of their levels. I had thought they were going to find away to pursue him home or lure him out, but instead they picked a different edge and tactic. They used their knowledge of the campaign settings as an edge to speed run experience to get back to level.
That was super fun.