Maybe an extension that would help it? Some tweak? I would e.g. open youtube on full screen but make the window smaller (non-fullscreen) and move it to a bottom right corner, so I would only see the video (similar to youtube app on Android) and then I would use a proprietary app to keep it always on top (e.g. Dexpot / autohotkey).
All I would need to hide is the panel with the tabs and buttons, addressbar with extensions. (just like when fullscreened)