I have to agree with at least stacking notebooks together! This would be a great feature.
For me it has nothing to do with competing apps but the simple fact so much work is done and ideas exchanged by PDF that they are almost like "notebooks" themselves. It would be nice if they could be imported as such.
For a Windows 10 device it seems reasonable there would be mouse-usable resize gadgets. I often times have used plumbago on my Surface while as a laptop and constantly reaching up to touch and manipulate things is much harder than ushering the touchpad or mouse. Maybe adding a resize gadget to one of the corners would would nicely?