Markdown, Markdown Extended, GLFM, Mediawiki Markup, Asciidoc, reStructuredText (RST), Pandoc, Mundimark, Djot, Creole, Textile, BBCode, Dendron, Pendown.
I bet at this point if you came up with a new completely independent standard it would probably match one of the ones already created. We mostly just have to pick one.
It's unfortunate that Markdown isn't better standardized so it could be extended to better meet the needs for man pages. (Do we need a new standard to rule them all? Yes, I'm thinking of the XKCD comic.)