I'm wondering how different languages implemented on top of this VM could be, especially at the semantic level.

Having different syntaxes on top of the same VM is nice and all the JVM languages show that there is room for a good variety and for different paradigms.

On the other hand, the choice of a VM sets some constraints while providing important features. Think for instance at the differences between the languages running on the JVM and the languages running on the BEAM (Erlang's VM).

I think there is a lot of untapped potential for innovative ideas at the VM-level but for some reason most of the effort goes into proposing new syntaxes that reuse the same concepts all the other languages are using.