I'm not sure how this would work in terms of the interface, but it would be great to have the ability to create fields that are applied to multiple content item types. This is particularly useful when creating fields that categorize the content and there are multiples types of content that are handled with the same structure (e.g. articles and downloadable documents). Obviously the primary categorization mechanism can be used for this, but there are often times when we need to categorize/organize content on several overlapping levels.