Based on the UX of other 'data' systems out there, it's actually quite odd that Vault only allows a single category to be assigned to an object.
I would like to see the current functionality morphed into a 'Primary Category' designation, with the added ability to tag objects with additional categories.
As a result, based on custom views, users could browse, view, seach and manipulate their data based on various Category views. This would go someway towards moving away from folder based navigation and viewing within Vault.
Lifecycle & Revision definitions would be pulled from the primary Category, but additional properties and their compliance settings could still be applied to the objects as a result of being associated with secondary Categories (If needed).
This idea has come about as a result of seeing M-Files recently. They have done a brilliant job of presenting data management to the end user.