Created attachment 2860
nasty looking mockup of sash idea
A useful feature, at least in my opinion, would be to have some kind of lockable sash, either vertically or horizontally, that acts to allow for a window to be maximised only up to the sash's coordinate.
This would be handy when using a widescreen monitor, and allow for a quicker workflow when using side-by-side apps without dual montior. However, I am unsure how one could make intially placing such a device relatively intuitive.
I attached a nasty looking screenshot to try to demonstrate what I am on about.
You might be looking for a tiling window manager (which xfwm4 is not)
Not quite, Its not one app per window -- apps can cross the border, they are just not allowed to straddle it. Its more like having the desktop switcher showing multiple desktops at the same time.