It *really* depends. I have written code that does this: markdown-ish auto-formatting inside/beside putative HTML. It is not trivial and unless you are willing to reject, or be highly opinionated about what to do with, “broken” content it gets harder; especially to be able to give meaningful feedback about what was wrong. I also think the world does not need another flavor of markdown or wikisyntax.
I think it’s worth trying to do. The biggest impediment for me to hacking on PM as-is is that I can’t just download and build a local copy where I can be stupid or experimental or whatever.