I wonder about the other direction - how much can you remove from Markdown until it's no longer useful enough for wide adoption?
How about:
one heading level only
no nesting (of lists or styles)
tables can only contain plain text (not even links!)
either indented or fenced code blocks, but not both
But keep the html escape hatch. If anyone needs one of the missing features, they can drop down to html. Just a bit more friction.