The information could be formulated in YAML front-matter e.g. as one of the following:
obsidian-ui-mode: editor
obsidian-ui-mode: render
obsidian-ui-mode: live # wysiwyg
…
Then it could be either ignored or respected by toggle in some Plug-in.
Files without this specified would behave as they do now.
Edit: reformulated in separate request: Defaut Edit/Preview mode per file in YAML front matter - #2 by malecjan