Replacement for GWT

I have heard about Vaadin (https://vaadin.com/home)

Vaadin is a covering around GWT giving you more flexibility to work and the same time maintaining the GWT like experience.

Otherwise this is a good compilation, you could pick what you might need:

https://github.com/jashkenas/coffeescript/wiki/List-of-languages-that-compile-to-JS

As a long time GWT user I am increasingly drawn to AngularJS. Obviously we're talking JavaScript rather than Java here, but coming from a Java background myself I'm finding JavaScript with AngularJS a decent alternative to GWT.