@jgm just to put some perspective on this, I’ve been surprised how often spoilers come up in typical internet posts. It may not be big in the Pandoc audience, but I assure you, for more non-techie conversational topics, spoilers come up wayyy more than I ever thought they would. They come up a lot.
I’d still get tables in first, but spoilers is certainly in my top 3 of potential markdown extensions that people actually use and ask for.
Heck you can kinda see that in how pervasive “rolling your own syntax” is for this use case! And it kinda sucks that they’re all over the place, no two sites seem to do it the same way 