If you have come this far, you have learned how to both write simple script-like solvers for a range of PDEs, and how to structure Python solvers using functions and unit tests. Solving a more complex PDE and writing a more full-featured PDE solver is not much harder and the first step is typically to write a solver for a stripped-down test case as a simple Python script. As the script matures and becomes more complex, it is time to think about design, in particular how to modularize the code and organize it into reusable pieces that can be used to build a flexible and extensible solver.
On the FEniCS web site you will find more extensive documentation, more example programs, and links to advanced solvers and applications written on top of FEniCS. Get inspired and develop your own solver for your favorite application, publish your code and share your knowledge with the FEniCS community and the world!
PS: Stay tuned for the FEniCS Tutorial Volume 2!