If you listen to a lot of fills in EDM music, notice that much of the time they're removing sounds as much as adding them. For example, in a busy track, if you end an 8-bar phrase with a kick and snare on the 4th count and NOTHING ELSE on that count -- bass, hats, chords, pads all drop out-- you have a pretty common fill. Combine that with an extra snare, a tom, or a sampled fill, and you're in business. It doesn't have to be drums, either, do the same thing with something musical.
I like making fills into mini-breaks too. The beat drops out on measure 7 or 8, and then a mini-buildup at the end of that measure restarts the beat.
Think of fills in terms of tension and release just like the big-picture arrangement.
