For WebGL and WebVR community this would be a big step backwards, making browser applications again second class citizen vs native apps.
And it's not like there aren't already strong enough protections in place. Try for example visiting this mock attack site:
http://feross.org/html5-fullscreen-api-attack/
In every browser I tried it was already obviously fake. It doesn't work already with current security tech:
1) browsers ask for fullscreen permission (with big unmissable dialogs)
2) emulated fake layout is very different from real layout (missing all per-user specific browser settings, e.g. bookmarks or extension buttons or any theme customizations, also font rendering looks different)
3) emulated fake browser UI doesn't respond to interactions in the same way as native UI