In my opinion the best thing in Gutenberg is the ability to save a post without reloading the page. I think the editor for posts (or pages or CPT) should use a modal, like the attachment editor in the media library, with instant navigation to previous/next post and button for instant close.
The current editor does use AJAX for autosave. It also stores content in sessionStorage of the browser, so you don’t lose your work.
I see nothing wrong with reloading the page.
We definitely don’t want a modal for the entire editor, due to
how Screen Options work,
how the page is identified for Help text,
the modals that are used by the editor(context forms, menus, charmap, links, search) and by core (media, help) and by plugins(could be anything),
fullscreen mode,
distraction free mode,
Text mode,
and the need to be able to access all the menu items while in the editor.
There is a PR already for adding a Next and Previous link on the editor page.
It is actually the worst thing in GB to be able to save without reloading the page (AJAX). Anyone who has worked with more than a couple sentences in a post will know why I say that, so I do not enter into further details.
I vote against any sort of “on the fly/ajax/js” features added in the backend.
If anything, the autosave is enough fancy already.
Things should work, not look nice but break at first attempt.