This looks pretty nice! Played around with it a bit on a few templates I had saved locally—I know you're not saying here that you support emails as well, but there is at least partial support (it seems to have issues with global color variables there.)
From a development point of view I'd be a little more interested in capturing module/editable area issues than having the ability to alter input values, especially when converting items from non-Marketo setups to Marketo.
Yeah, I debated on building that into but considering the tool is mainly meant for developers, I figured it would be just as simple for them to edit it in the HTML file.
If Marketo wanted to throw some money at this. I would absolutely build all of the template features and behaviors for email and landing pages into the extension in order to develop out side of Marketo.