Yeah, I struggle with the same stuff. Either I have too much going on in a track, or not enough.
That's why I mentioned the sustained notes/pads, since I've found that even at a barely audible volume a sustained sound working as a pad in the background helps to make a track feel more full/not hollow, even if you don't have a lot of instruments playing.
A friend gave me that tip when he found one of my tracks felt too hollow, and I've found that to be a really good thing to keep in mind when arranging a track.