So perhaps the best starting point would be to look around the existing desktop environments and seeing if any of them could be suitable on a keyboardless touchscreen device with minimal changes.
The second thing I would suggest naturally comes from the first anyway but I will spell it out just in case. Focus on function, not the form. ... Good old Palm did that just right: the dot was a dot without a shift in both the letter and the number mode. Why? Because they cared about workflow down to the minute details like that.