Perhaps there could be an X11 "launcher" much like the Failsafe Terminal mode, in which it launches a super-lean window manager with just that app (and maybe a terminal 'r something so you can launch more).
In fact, if I recall correctly, X11 has its own window manager built in.
Or maybe that's a stupid idea, and the GMenu2x should just be (part of) a super-lean window manager to begin with.
X11 doesn't have a window manager. End the window manager process and suddenly your windows have no borders and you can't move them. even with the linux alt key tricks.
But yes. A failsafe launcher in X11 is a good idea.
So we default to X11 with NOTHING else but a gmenu2x style launcher as default (or something with similar basic functionality). We can from there launch just games, or programs and a window manager and task bar. Then have an option to kill the window manager and task bar if you need to free up memory again for a game.
Those who want more from the start can add a window manager and task bar etc to the X11 startup script.