It would be nice to be able to compare two XML files
from within the IDE, and view the output graphically.
There are various XML diff programs out there; some
free, some not. Maybe they could be pluggable. Or else
one free one could be selected and used in Treebeard.
Going along with this it might also be nice to be able to
apply patches, merge versions etc.; but that's above
and beyond. Simply being able to compare two XML
documents and view the diffs would be valuable in itself.