The most interesting language design I have seen is Noether. It is a language built on multiple layers. Inner layers guarantee more invariants (say, immutability) but forbids constructs you may want to use. Outer layers are more expressive but guarantee less invariants. Importantly, inner layers are not different languages but sublanguages of outer layers, and inner layers keep guaranteeing invariants even when used in outer layers.

http://www.infoq.com/presentations/noether