Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. — Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

Future Work

These are just some ideas. There is absolutely no guarantee that any of them will ever be implemented.

Use SBO.
Allow more control over vtable layout.
Attempt to reuse sub-tables in conversions.
Allow "dynamic_cast". This requires creating a global registry of concept mappings.
Optimize the compile-time cost.