With thanks to Esko Luontola I now have a pet project. It’s called Dimdwarf. A direct descendant of RedDwarf but with some additional finery (the biggest difference being that managed references are invisible to the developer).
The project was written in Java (some helper scripts with Ruby) but all the new code is written in Scala. Thus, I got a perfect opportunity to hone my skills. Also, the project architecture contains almost all the things I have wanted to work with. It has Software Transactional Memory, it uses Guice for dependency injection, it is developed purely with TDD. No exceptions to that. And statics are forbidden! It is designed to be scalable, aimed for clusters and much more. See the project pages for more information.
Now, all I need is more time. Could someone add more hours to my day?