This is a nice little medium between fullscreen and windowed, it'd be a great feature to add (I have 2 monitors)

i don't see why not, borderless looks alot nicer too.

supported

Also, maybe an option to have your mouse locked into the window.

The game would have to be moved off SDL first, it's pretty terrible at borderless fullscreen.
SDL can do it, but there's a pretty bad performance hit & mouse cursors vanish when not moving. Pretty useless really.

Would be nice, but yeah, it'll mean a big core change before it'd happen unless you hate yourself and want the above issues.