I'd love to see XML Copy Editor automatically populate elements with required attributes and sub-elements based on the Schema or DTD file named in the working document.
For instance, when a new img element is created, an alt attribute would be placed in the opening tag, because the XHTML DTD requires this. When a new ul element is created, a li sub-element is placed within it, because a list element requires list items. (At least, I'm pretty sure that the DTD specifies that.)
This would save a huge amount of time when working on XML documents with rigorous schemas, as it saves you from typing the same old sub-parts yourself every time.