On the German-language Wikipedia, we have Vorlage:Webarchiv, which supports three different web archives, e.g.

{{Webarchiv | url=http://de.wikipedia.org | wayback=20020921204942}}
{{Webarchiv | url=http://en.wikipedia.org | webciteID=1218580588013234}}
{{Webarchiv | url=http://de.wikipedia.org | archive-is=20020921204942}}

Since all three archive-ID fields have to be tagged as required in order to be visible to a user inserting such a template, the VisualEditor adds two empty named parameters, even though logically, only one archive-ID can be non-empty:

{{Webarchiv | url=http://de.wikipedia.org | wayback=20020921204942 | webciteID= | archive-is= }}

A similar problem was raised today here, where a template has two different spellings of the parameter ''Größe''/''Grösse'' to be used for German/Austrian and Swiss articles, respectively.

In both these cases, it would be useful if there was a way in TemplateData to specify that only one of these parameters can be set, resulting in the other parameters not being added.

Another usecase was already suggested in a comment to a different task:

In T51743#1727808, @Trizek-WMF wrote:

Based on a discussion on frwp, it appears that some fields should be conditionally displayed TemplateData.

For example, on an infobox, if there is no image, you don't need to suggest fields like "image caption" or "image size" on TemplateData.
Is it possible to have conditional displaying like that, @Jdforrester-WMF?