I have an idea: i often work with blender and inkscape, and i realy like resize objects in this programs. For example - draw rectangle in inkscape and try resize it with Shift =) I don't know is this good idea to resize windows or not, but i like this style. :)
It's certainly doable but I honestly see little benefit in this for a window manager.
If you add to this that some apps specify aspect ratio, size increments and window snapping, it may become slightly more complicated.
You may want to experiment with a patch though and contribute it.