Honestly it's good to leave in some empty space.
That way your song will sound more dynamic and if there's too much empty space, you can take care of it by mastering properly to fill those gaps.
More layers like pads, extra leads or sound effects like white noise or other fx can always fill up the song.
I like to think of the song as a big bottle. All your instruments are big rocks that fill up the bottle nicely but there are a lot of gaps left in between.
These additional layers and effects are like sand that smoothly fills up those gaps and makes it a full bottle aka a full song.
That's how I see it =D