Don’t like !!: , potentially confusing. Plus is inconsistent with ‘code fencing’ syntax in CommonMark, which is visually similar.
Don’t see how that’s any harder to use. You don’t have to have all () [] {} elements since !nameOfExt is the base trigger word. You only need to add elements as you need, much like how english has core based words that are used often, and a person will learn many more vocabularies to handle edge cases. Thus for your examples it would be like:
!spoiler[ Click me to expand ]( Hidden text here )
!sidebarElement[ home ]( #home )
!notice[ This is an official notice to everyone ]{ .noticeclass }
If you need syntactic sugar, here is some extension to the above syntax for notice extension. Only supports strings however.
!notice:"This is an official notice to everyone" { .noticeclass }
alternatively without : (Omitting noticeclass class as well):
!notice "This is an official notice to everyone"
No recommended due to potential edge case, when wanting to use extensions without arguments.
e.g.
!randomQuote
vs
!randomQuote "this is not part of !randomQuote"
tl;dr: It is flexible enough for both block and inline. Can consider !name: " ... " or !name " ... " as an optional alternative to !name[] for natural message .