I’ll gladly admit - I poked and scanned through the thread there, but I did not read the whole thing. Still, I think (I hope) I got an idea of what you’re saying.
I’m completely for using whatever syntax everyone wants. I wrote something (the stuff above), and it’s what I’ll use for now (it’ll work for me for now), but it’s non-standard. I would rather know the proper standard for those, or have a standard.
I like blockquotes - I think they bring value to markdown. I also think they could be extended in a generic fashion somehow, and by doing so, you could in turn have that extra bit support a lot of stuff. For example, moving away from alert/warnings, spoilers could simply be:
spoiler> This is a spoiler.
And of course, once again, that’s sticking with the standard I already implemented. Whatever is decided is fine - I can re-implementing what I already did. I tried to do them following the current blockquote as much as possible, and tried stealing influence from the opening/closing code blocks - but I’m happy to admit there are lots of other ideas that probably fit better.
Of course, as I believe you mentioned in that other thread, I do like the idea of using something other than the below example that’s thrown around in that thread, as it’s not really extensible:
!> This is a spoiler
Also, of course, if I should just merge this into the other thread you linked, I can. I’ll be honest, I’m new to this system, but I’m happy to do whatever is proper per style guidelines, and happy to figure out how to do that on my own. 
I’m also sorry that I did not find that thread to being with 
but I’ll lean on whatever grace that newcomers get around here.