I read this, and understood some. Here's my contribution: Because I am a monster, and because DW allows it, I'll often specify !markdown in a post and then go ahead and use an unholy mix of HTML and Markdown. Would that mixed usage--perhaps for older posts only--be covered by one of your format modules?

Also I use Semagic to post to DW, so I wouldn't be able to select a format for each post. Instead of having the site guess what format to use, would it be possible to create a "!markdown" like command to put at the head of a post to indicate the format? Or maybe have an option in settings where a user can choose a default format for new posts.