I'd like to see the functionality to define custom data types or content types.
Right now Custom Fields does give the ability to define a custom entry screen or category or page. In many cases I want discreet entry types, so, I'd like to be able to have one set of fields on one entry type (menu: Create: Entry) and a different set of fields on another entry type (menu: Create: Bio Entry).

I guess the only way to add stuff to the "Create" menu is via a plugin. A lot of CMS's provide the ability to create these custom "data definitions" and then easily present them in the CMS.

... user since v2. Great application.