A common feature/support request runs along the lines of "How do I filter this fields equivalence to this other field". For example, the most common variant of this is that a user may want all nodes with a CCK field equal to a user's profile field.
We could accomplish this by allowing the 'value' field to have an additional selector. Instead of just the typical value field, we could add all fields that are in the query (we would probably have to except pre_render and non-querying fields since they're pulling data differently) and simply put that fields field_alias into the query.
Obviously, this could not possibly work on exposed filters, but that actually makes it easier. If a filter is not exposed, we could put in a radio button that allows the user to select from fixed value or field. The field shows a select list. It might require some retooling of the existing handlers to allow this but perhaps an alternate query() method would be the easiest way to minimalize the impact on future development.
FAILED: [[SimpleTest]]: [PHP 5.4 MySQL] 69,630 pass(es), 2 fail(s), and 0 exception(s).
PASSED: [[SimpleTest]]: [MySQL] 59,208 pass(es).
PASSED: [[SimpleTest]]: [MySQL] 59,457 pass(es).