The idea is to explore all the features that Groovy offers in this space (closures, command syntax, type safety) while also considering other factors as to whether a DSL is worth creating in the first place.
If you want to join in, we recommend you have at least an intermediate level of Groovy knowledge so that you can follow along with the discussions. Building a DSL often involves fairly advanced Groovy usage.
The DSL will necessarily have to be very limited in scope, but that will give us greater opportunity to explore a wider range of topics. We'll decide on what DSL to attempt closer to the time, but if you have any suggestions please add them to the comments!
Peter is a long time Java developer and one of the core Grails committers. He has been developing with Grails since version 0.2 (give or take 0.1). He created the GWT Grails plugin as an exercise to see whether it was possible, but it has since grown into something that can support serious development.