The Geom$optional_aes and Stat$optional_aes fields seem redundant to me.
Instead of having these fields, we can declare default_aes = aes(optional_aesthetic = NULL).
While I would like to see these fields go in their entirety, that seems like backward incompatible, so instead I propose to discourage these fields. We can (i) remove any references to that field in the extension vignette (ii) ensure we adopt the NULL-aesthetic approach within ggplot2 and (iii) check if the ggplot book would need to be adapted.
If we're in a violent mood we can start throwing deprecation warnings about that field, but it seems sensible to discourage first and reconsider the next release cycle.