Good music is all about dynamics. One way I am able to get my music to have a wider range is having many different types of instruments. Instead of just having synths, I love to mix in organic acoustic sounds, physical modeling synths, and square wave synths. It gives the track interesting sounds and feels more dynamic and full.
One instrument I love to mix in my synth-heavy tracks is this Celtic Harp I created. It’s so clean and pops out the mix. It can add a delicate flavor to an otherwise heavy track.
This instrument is for free / by donation. Check out the video below and a link to the download.