Hi,
If you are referring to the plane that you sketch upon and subsequently extrude your model then technically you
already can select those planes with the three planes at the Origin. I would suggest that the reason that you cannot
select the sketch plane from the view cube is that you can select many more planes than simply the six primary
views.
Sketches and extrusions can be placed on any plane and planes can be generated through the top menu selections
where such things as offset planes and planes at an angle can be created from geometry you have created. It is
possible to create completely custom planes with three points in space that the view cube simply would not be
able to handle.
The current system is also more or less a common standard used across many software packages and fusion wants
to be different and unique, but not to the expense of new users being confused because the software is too unique.
Cheers
Andrew