+5000 to Jay's answer.
It's important to think like a composer - arrangement, textures, brightness/contrast, pace, space and emotion (the most important IMO). What do you/the director/audience want to feel after watching/listening to this?
I usually start abstract pieces by creating a bed of sounds - maybe a pad or something rhythmic that goes with the pace of the visuals. I then start filling in sounds instinctively - I allow myself to overdo things and experiment without any judgement. Then (the most important part for me), I take a step back and start scrutinizing what I've done (must be careful here, it's easy to hate what you create because it's never good enough) and begin to remove sounds. If I mute a sound and don't find myself missing it, I leave it out.
I then pay attention to the feeling/emotion that the piece invokes in me. At this point I also think about the rhythm and pace. Think about breaks and rhythmic contrasts. The trick sometimes is to make it happen when it's least expected.
Also, when creating abstract designs to picture, it's important to have some sounds sync/follow movement on the screen. It helps the sound be a part of the visuals - instead of two discrete streams (unless intended otherwise).
If you find yourself short of ideas, pick a couple of random sounds from your library and spread them across the timelines (maybe even insert an fx plugin on some of the tracks). It might inspire you to think differently.
It might also help to set up a metronome that goes with the pace of the film. Switch to grid mode and look at your time line visually and fill in sounds.
There are many many many ways to approach this. As long as you are staying true to the intended 'audio-vision' you should be ok.