What: Ability to explicitly define the domain of a data element.
Why: Tableau currently allows us to define the type of a data element, but not restrict the domain of possible values. Still, it happens frequently that we know limitations on possible values. Even if the dataset currently doesn't contain every possible values or every possible combination of values. Being able to define the domain on a data element level would allow to show the full range of possible values in legends, filters, axis, ... See list of related ideas below, as well as various comments by kettan and Jonathan Drummey .
Proposed solution: Allow the user to explicitly define the domain of a variable. This would not replace the data type definition but would come on top of it. The domain would then be used as default range for axes and for filters. See simplistic mspaint mockup attached. This type of domain definition already more or less exists for parameters and also on import filters, so adding a similar dialog box for regular attributes isn't much of a stretch.
Additionally, I would really like the ability to save such custom data domain definitions on application level across multiple workbooks. If we could assign e.g. colors and sort orders at a data domain level across workbooks, it would be make it easier for us to be consistent across all our reports.