With T76778 Flow will respect an existing page's content model, 'flow-board' or 'wikitext', and there will be ways(API call and Special:Flowify page) to change content model. But how do we determine if a new page is Flow or not? It seems we still need a hint

• when you visit a new page, do you see empty wikitext or an empty Flow board?
• if a bot comes along and tries to add wikitext or make a Flow operation, do we let it? Do we consider it changing the contentmodel?

If we keep the notion of "Pages in this namespace should be Flow-enabled by default", we have to decide whether subpages are Flowified as well (T76197) and also what to do with subpages during namespace conversion (T76772).