Is it really possible to create a markup which will suit everyone? I don’t think so. The problem with markdown, or any of the hunders of markdown derivatives, is it’s very static: you can’t expand it without “forking” it.
Will commonmark be able to please a scientist? A novel writer? A blogger? A web developper? Won’t it miss a critical feature?
Before markdown was invented, there were other markup languages. I choose txt2tags many years ago because even if it’s not perfect, the basic syntax is very logical and easy to learn (most of the reasoning from the creole website are valid for txt2tags : http://www.wikicreole.org/wiki/Reasoning), and most of all the killer feature is you can create your own specifications, which will be written in the header of your document, so other people will read what modifications or additions were made.
you can see lines starting with “preproc”, which will use regex to expand the original markup. This way, with only a dozen of lines, I can make a txt2tags document which can read most of the markdown syntax.
I think therefore if you choose to create a Common Markup, you should leave a way for developpers to easily customise it for their own purposes (read: regex).