I've been using Snapshots quite a bit for my recent projects and I think there could be some small improvements to make them even more useful.
Drag Snapshots from Pages & Masters Panel
Press CMD key and drag a page or master to to create a Snapshot - similar behavior to reference rectangles, just with hold a CMD key.
"Click to open reference page" checkmark
Add the above checkmark to the Snapshot properties to be able to create an automatic link to the reference page.
This makes sense when you use Snapshots to illustrate flows.
Apply correct proportions by default
(Similar behavior to image D&D.) It quite cumbersome to resize a Snapshot every time you use it.
No padding by default
Please remove the default 5px padding. Scale them by hand (to get rid of the whitespace) and having a margin is quite painful.
CMD + doubleclick to open reference page
This would be a nice addition when creating flows in Axure. Then you don't have to scroll through your pages or masters list to open the correct reference. This functionality should also be added to reference rectangles.