Most serves are too simplistic. They get the job done, but simply aren't done with real trickiness in mind. For example, a player serves backspin with a backspin motion. They may learn variations, such as a side-backspin serve, where the racket travels in a different direction in hitting the ball, or a pure sidespin. But the direction of the racket at contact is somewhat easy to see.
Why make it easy on the opponent? Sometimes, just before or after contact, change the direction of your racket so your opponent has to pick up whether you hit the ball before or after the change of direction. Better still, learn to hit it either before or after the change of direction, giving different spins with the same motion. You can also develop a semicircular serving motion, so the opponent has to figure out where in that semicircle you spun the ball - there will be different spins depending on when you hit the ball.
Developing serves where you hit the ball on different parts of the swing are tricky. What's easy is doing fast changes of direction just before or after contract, where the serve itself is seemingly simple, but the opponent is caught off guard by the change of direction of the racket. For example, with a forehand pendulum serve, right after contact pull the racket away from you, as if doing a reverse pendulum serve. Or set up to do a reverse pendulum serve (with racket close to you, with the motion going away from you), but "miss" the ball on the out-swing, and then do a regular pendulum serve, with the racket hitting the ball moving toward you. In both cases, it's a forehand pendulum serve, but you've added deception to make things harder for the receiver. Isn't that the goal?
Some players do this type of thing on every serve; some do it as a "surprise" to catch the opponent off guard. Try it out and you'll be surprised how many "free" points you get!