I kept thinking about the last part. Scala is a really nice language. The GWT route was to create a Java compiler to generate JavaScript. Doing the same for Scala seems like a huge effort. But what about this option? It is a little bit crazy though.

What if you use Scala to create a DSL to create JavaScript RIAs? The DSL can be the framework that will give structure to the RIA. Then you use JavaScript to fill in the blanks.