File linking seems to be a common request as it fits several design workflows, so I +1 that, yet, even more helpful would be to be able to "reference" shapes, paths, colors, groups, etc. In other words, have a group that is used several times in the document, but when changing one all others change.
This features would be very helpful, for example, for UI design, where it is common to have the same control used in several places, or where changing colors or fonts of several parts of they UI is common to be able to see things in context.
Another example is a style, say I assign a style to several shapes, and then I want all the shapes that use that style to get updated when I update the style, stuff like that.
In summary, it's just like file linking, but for internal parameters... aliases? proxies? placeholders?
j