Since you have now quite a bit of experiance with rectangles I think that stepping up to something that will help you grow as a quilter would be a perfect match.
What about a sampler?
This is the book that I used to make my first quilt.
The book is layed out so you start with block one and as you go through each block new skills are introduced. So when you end you have done basic rail to half square, set in seams, applique. All my points are not perfect on this quilt but it is a beauty and I learned so much.
http://www.amazon.com/Learning-Quilt.../dp/1574867202