+1, I would dearly love the ability to nest symbols.
Symbols are great:
My main pages should be symbols.
My navbar and sidebar should be symbols.
My common controls and widgets should be symbols.
Nested symbols follow naturally:
My main pages contain navbars and sidebars.
All of the above contain common controls.
I'm sure your dev team is made of people smarter than I am, but I just want to make sure I've helped this feature along in every way I can:
Internally, your symbols-referencing-symbols structure forms a directed graph. Use a graph traversal algorithm to efficiently figure out which symbols need updating. But first (to avoid an infinite loop) use a cycle detection algorithm to make sure symbols don't reference themselves in any way.