Very interesting proposal! A few comments:
Emphasis
Couldn’t agree more. The current markdown rules are confusing to explain to new users as well.
Though before making a definite decision, I would love to know current emphasis usage statistics. Are there any usage numbers of markdown documents in the wild?
(Not so sure about fan~_tas_~tic though, but I guess why not?)
Indented code blocks and lists
To remain somewhat more backwards compatible to CommonMark, instead of getting rid of indented code blocks entirely, one could also simply disallow them within lists and blockquotes and such, but keep allowing them when not nested in another element (which should account for 99% of existing uses).
Raw HTML
{=html} is great, but what about using Markdown inside HTML? Like:
<aside>
my _great_ text
</aside>
Maybe if we had a generic block container (like the ::: in pandoc), Markdown inside HTML wouldn’t really be needed anymore.
Attributes
As you probably know, I’m all in on attributes. (Attribute discussion on this forum.) The specifics for different elements are a bit trickier to figure out (e.g. paragraphs, lists, list items), but I can see how placing the attribute before block elements, as you propose, might help in parsing.
several may be used (and will then be combined):
Not sure about that, isn’t the following simpler?
{warning #mywarning}
Dreaming?
Interestingly, most of the proposed changes are things only markdown-power-users would notice anyway. For example, most people just fiddle with lists until it’s right in the preview.
However, Emphasis and Raw HTML are two things almost everyone who has come across markdown somewhere is familiar with and would probably find annoying if it doesn’t work the way he/she expects anymore. Not sure what that means though… maybe if it weren’t for those two changes it could even pass through as “CommonMark v2”?