By unused frequencies I mean... it's a little bit hard to explain, for me but I'll do my best:
So, when you have say, a rock song, typically you'll have the following setup at the very least: bass, drums, guitar. In your song's case it's bass, percussion/drums, guitar, keyboard.
So the instruments would occupy the following ranges:
Drums: varies/range agnostic
Bass: bass range
Guitar: mid range
Keyboard: mid range/high range
Then we have to consider the way the instruments are being played:
Guitar: not a lot of reverb (+potential mid range that could be used)
Keyboard: this is the big one: not a lot of reverb, short, "pluck-style" release. (+ a lot of potential mid and high range that could be filled should you want to)
So in this case, I see 2 things you could do if you'd like:
Option no 1 would be to have some strings or pad in the background, either overlaying the keyboard lines or playing their own harmonies.
Option no 2 would be to add more reverb to some instruments. The way you have it set up allows you some freedom in adding a bit more reverb to the drums, perhaps adding some more percussive elements on top of them. Not only that, but you could choose to add more reverb and delay to the keyboard line and the guitar. Your current mix would allow for it.
I hope that helped. I'm not always the best at explaining the technical side of music.