One of the options in the Define Value List dialog would allow use to invoke the calculation engine to generate the list based on a formula.
This would combine the dynamic quality of a value list based on a field and the control of a manually-defined value list.
ExecuteSQL() in this case would give us very fine control over related values without having to create table occurrences and relationships just for that task. ORDER BY would give us finer control over sort order in these cases, too.
BONUS FEATURE: In value lists which draw from related tables, have FileMaker automatically store the related key field value (specified in the Define Value List dialog), which we could then retrieve using Get ( ValueListItemKey ) whenever a value list item were selected. This would eliminate the need for kludgy "Show only second value" value lists and such.
EDIT: Just realized that this would open up a world of "virtual tables" if we could define a value list based on SQL and then use ValueListItems () to retrieve the table.