Currently the ability to set a conditionally default (aka a default based on which user is logged in) that can later be overwritten is not in Tableau.
There are some possible workarounds. In this case you might want to try option 3 in Set an Overridable Dynamic Default Selection in a Filter. You could save the workbook with "default" in the category parameter, and then the end user could type in another category.
The Tableau website has an Ideas section where customers can add, comment, and vote on features they would like to see in the product. Votes are used to help prioritize new features. Our Development team also loves to see everyone's use cases in the comments, which helps them better design features that will meet the most possible needs.
These are the related Ideas I found:
Hope this helps