What can I do with miUML metamodel?

You can use the metamodel as the foundation for designing model editors, model level debuggers, model to code compilers, import/export tools and any other MDA utilities.

If you want to build or adapt a tool, such as a graphical model editor, it can be either open source or proprietary as long as the metamodel library (models and API) source, along with any of your modifications, remains freely available to everyone.

The metamodel is also the ultimate reference to the executable modeling rules.

The models may also serve as a learning resource.  Advanced modeling practices such as merged referential attributes, stable generalization partitions, constrained loops, carefully named association perspectives, use of domain specific types, strategic use of specification classes, and strong documentation are few topics you can explore when you curl up with your metamodel descriptions at bedtime.