Badass Designer roadmap. Please at least review how Fireworks implements shared layers across states (which I have been misusing as pages for years). It's the most powerful implementation of a layer - sublayer - "page/mock" relationship I've seen; suppose you need a main menu across 10 pages/mocks, but you have a 20-page doc. You simply “share layer to states”, then hide it from those pages you don't want it on. All updates to that layer appear on all states (again, as pages) without absurd asset duplication cause by art boards. If you need unique menus, you can simply hide that layer and make another, or unshare the layer and decide which pages get that content. It works like a symbol but with the advantage of edit in place and locking certain sublayers if needed.
