Stories are made of two things:
The things that get in their way.
A good antagonist is both. Sometimes the conflict comes from the situation or the setting, but most of the time it’s a person. Some of the greatest characters of all time have been the antagonists – think of the Joker, the Master and Loki for some recent examples. The best antagonists are at least a little sympathetic. They’re not doing what they’re doing because they’re evil and it drives the plot forwards, they are driving the plot forwards because what they want conflicts with what the protagonist wants.
Admittedly, the Master and Loki want to rule the world, which conflicts with what all the other characters want, and there is a showdown I want to see.
Ideally, you should know your antagonist as well as you know your protagonist. You should be able to flip the story and tell it from their point of view and reverse the roles, because now it’s the protagonist who is preventing them from getting what they want.
So who is your antagonist, what do they want and why do they want it? How strong is their driving force, and what lengths are they going to be willing to go to? Because whatever they’re willing to do, that’s what your protagonist is going to have to overcome.