The stepladder works well as a stepladder (who would have guessed!).
If I was going to build this project again, I would see if I could make deeper steps. I was limited by the quantity of wood I had on hand. It works fine with the shallow steps, but deeper steps would be nice.
Now that the stepladder is done, I can see it would have worked fine with two steps instead of three. Not a big deal though.
A slightly wider stepladder would be nice too, it would be that much more stable.
Getting the stepladder to fold flat properly would have been nice. Following the plan more closely would have fixed that. Alternatively, you could make a full-size or scale model of the folding joint using scrap, to make sure that the folding parts will sit together properly when closed.
I think you could build a reasonably tall ladder using a similar layout, but you would want to angle the legs outwards, and then you would need to use something other than dado joints for the steps.
I am quite keen to make more projects with moving parts like this, it is neat being able to change the size of the stepladder for storage. Maybe you could make a fold-out toolbox ... or a transforming coffee table
... the possibilities are endless. :-)