After you have created a melody, chords, and other backing sounds and your music still sounds unfinished as in still lacking depth, try adding a backing melody that counters your main melody's rhythm. An example of this would be where the main melody plays long notes while at the same time the countermelody adds extra notes.

The countermelody doesn't clash with the main melody. Its purpose is to support by making the main melody sound even more interesting. Therefore, the countermelody can rest some times for the main melody to move or harmonize with it by playing the same rhythm. Imitation is another great technique.

For this song, I have created two different versions of a countermelody. One is more obvious than the other in the way it moves against the main melody, just playing more notes. I also added a countermelody 2 that imitates or harmonizes.

https://youtu.be/aox45sjoE6s