Minimalism is all about change by gradual transformation. Picture a minute or an hour hand on a clock. Whereas with most music (symbolised by the second hand) you easily perceive each change and development at almost every moment in time, minimalism is like that minute or hour hand - you really can't see it move if you concentrate on it, yet after a certain amount of time, you might find yourself realising that it moved from one point to the next by imperceptible gradual motion. Transformations in minimalist music can be thought of like this, and pieces in this genre strive for the essence of this whole "gradient" idea.
So what you might want to do, is figure out what you'd like the beginning to be like, and what you'd like your end point to be like. Maybe they will be completely different, not just by chord name, but by motivic development, patterns, rhythms, sonorities, range, dynamic, layering of different elements, anything really. Then you take this beginning and end and paint a line from one to the other in music, gradually transforming until you've reached your goal. Whenever you think things might be getting old and boring, add something new, or begin another transformation, make it subtle, the listener will catch on and perceive it as a story that slowly progresses, yearning for a resolution. Some music goes from a start to an end, and back to the start again, creating a sort of journey to "far away lands". Some of it just goes through endless transformations in a free form, it all depends on what you want to do with it.
It isn't necessarily that you have to make your material more interesting by adding to it, but that you transform it into something else. Why do you think Philip Glass used the word "Metamorphasis?" Also, when in doubt, go listen to some real minimalism, get inspired. Copy ideas - maybe you'll soon develop your own idea of what exactly minimalism should be.
Hope this helps any. :)