Our software Construct 3 (https://editor.construct.net/) I think meets most of these points. It runs offline, and we never have access to the users project files. You can save/load locally, and it runs in the browser. Game project files are zips with JSON + raw asset files. No syncing with the server needed so it is fast, and a design mistake that is severely hampering some of our competitors!
I'm not entirely sure how supporting collaboration in real time belongs on this list. Seems like a nice to have that isn't really related to the rest of the list.
I'm not entirely sure how supporting collaboration in real time belongs on this list. Seems like a nice to have that isn't really related to the rest of the list.