We’ll need to decide what markup flavor will be used and whether we develop our own editor or use an existing one.
We want not only to be able to import posts from Stack Exchange, but also to make it easy for users to migrate. Therefore the markup language needs to be close to the one used on Stack Exchange. It doesn’t have to be 100% identical, but it needs to be sufficiently familiar for users to keep their habits.
The most likely candidate is CommonMark. It’s a flavor of Markdown which is not strictly identical to the one used on Stack Exchange, but close enough for translation to be not too onerous, and more importantly close enough that most users wouldn’t notice the difference. Stack Exchange has been planning to move to CommonMark for a while. Many tools to parse and translate CommonMark exist.
Or we may simply adopt the Stack Exchange markdown flavor. StackEdit is an open source browser widget for SE Markdown.