Today, any Alteryx tool with "Select" functionality has an option for "Dynamic or Unknown Fields" which, when checked, allows any new fields to pass through that tool. This is a great function for most of the tools as you can allow workflow updates to pass through the tool without issue.
However, in the Join tool, there are some use cases where there is NEVER a reason to pass new fields from one side or the other into the tool, but you might still want new fields from a primary process. Examples being something like a lookup/cross-reference to do an inclusive join, where adding new fields to the lookup might inadvertently pass these downstream. Having the option to only allow unknown fields from one side through would greatly enhance this output.