Hi sammycat!
After mastering Low Impact Step, I would suggest you try some of Cathe's older step routines from either her Classics DVD or Cardio Hits. I don't have the Classics, but I do enjoy her Cardio Hits which has Step Fit, Step Works and Power Max plus an ab routine from Pure Strength. Power Max has more athletic choreography, Step Works is the most dancy and Step Fit is somewhere in the middle and has a macarena cooldown. As far as which series to try next, I would get her Body Blast Series which is intermediate/advanced. I have almost all of Cathe except for her earlier series like CTX and the Classics. I love complex choreography so my favorite of Cathe's step is Rhythmic Step and then Step Blast (Step Blast is from the Body Blast series, it is complex but hopefully once you've mastered Cardio Hits, you will be ready to try and tackle Step Blast). I really enjoy the Body Blast series, the music is great and there are so many premixes to choose from. These workouts are a lot of fun for me. The only other workouts I don't have are her intermediate workouts like High Step Circuit, Basic Step and Body Fusion and Low Impact Step).
Welcome to Catheland.
Marcy