You know what? We had that. Markdown was modelled after it.
I would also state that Markdown itself is completely unsuitable for the purpose. You’d need to design something new which might look very similar to Markdown, but which would have basically no shared behaviour with even CommonMark as regards parsing, since you don’t want HTML to be a thing. Markdown itself is seriously compromised by its HTML basis.
I can’t actually think of a single comparatively-mainstream messenger that uses even a variant of Markdown; rather, they use their own lightweight markup languages <https://en.wikipedia.org/wiki/Lightweight_markup_language> that are very clearly incompatible with Markdown. (It’s also often a frontend editing feature that gets turned into something like HTML after that.)
I would also state that Markdown itself is completely unsuitable for the purpose. You’d need to design something new which might look very similar to Markdown, but which would have basically no shared behaviour with even CommonMark as regards parsing, since you don’t want HTML to be a thing. Markdown itself is seriously compromised by its HTML basis.
I can’t actually think of a single comparatively-mainstream messenger that uses even a variant of Markdown; rather, they use their own lightweight markup languages <https://en.wikipedia.org/wiki/Lightweight_markup_language> that are very clearly incompatible with Markdown. (It’s also often a frontend editing feature that gets turned into something like HTML after that.)