Perhaps when the [[WP:Tutorial]] on enwiki is updated for VE, we will want to enable VE on the tutorial sandboxes.
Also, when VE is enabled for anonymous editors, we might want to turn on VE in the enwiki [[WP:Sandbox]], because otherwise anons won't have anywhere to play with VE (they cannot create user pages).
Perhaps a category (similar to the way AFTv5 does it) or MediaWiki page (like Echo blacklist) would be the way to do it.
Version: unspecified
Severity: enhancement
For the record: on ptwiki we also have a few sandboxes on Project namespace:
[[pt:Special:PrefixIndex/WP:Página_de_testes/]]
While bug 55900 now allows logged-out users to invoke VE in the namespaces where it is already available, in the meantime I hope communities created temporary sandboxes in one of said namespaces :) (The Italian one did, it's basically a subpage of mine).
If we ever do it, I think it should be done using a double-underscore keyword placed on the page (or in a transcluded template), e.g. __VISUALEDITOR__. This would be analogous to e.g. the __NEWSECTIONLINK__ keyword.
Also, note that the &veaction=edit URL parameter can now also be used to open VE on pages where it's normally not available (T221892). You can put a big blue button or something with such a link on sandbox pages, see e.g. https://de.wikipedia.org/wiki/Wikipedia:Spielwiese.
This is a huge pain point, newbies cannot try out the tool in the sandbox that was created specifically for them…
This has three issues: