In no particular order:
* An an option to auto-resize hashes when they are a certain amount full.
* Add scope push/pop hooks, so that we can clean up how s2 hooks into the scope system.
* Support the special infinity and NaN values. In native C these require C99, and we currently only require C99 for the fixed-size integer types and printf/scanf format specifiers.
* See if we can implement property accessor interceptors in the core or whether that needs to be farmed out to the client (where it would require more property search operations). There's been [/timeline?r=accessor-interceptors|some experimentation] done with this, but that isn't the right approach.