We currently reserve 3 words of inline storage for existential containers. We may want to reevaluate this decision for codifying it. This might be too small for important patterns. e.g. NSRect does not fit.
We also need to consider whether we want to stack allocate local existential boxes (when the type doesn't fit in the inline buffer).
This is a secondary ABI concern. i.e. it would be nice to reevaluate the current design before the final ABI lock down.