To be clear, you do mean a “tag” rather than “category”, right? A category will appear as a colored section visible on the home page of this forum. I ask because it sounds like you want it to be a “place” to post things to the wishlist. I’d like to restrict the number of categories in this forum. (While on the topic now, I wonder if we can even remove or rethink our current categories.)
I think it seems like a good idea to exist as a “tag” however. No matter the category in which a topic is raised, it can be tagged as having an wishlist item. My only concern is that this tag might be overloaded to mean something else, and then become meaningless. It would be nice to have tags listed on the home page, like categories are.