As a content developer, user, and buyer, I must say the "Default things" filter needs to be added. Let me explain why :)

If we don't know which items are custom, there is a high possibility that we might unknowingly and unintentionally use another user's work in our own. For example, if we are creating characters and we don't know which default parameters to use, we might unintentionally use another person's characters to build our own and modify them. When we add these to the marketplace, it will show that the base content is from another user. Consequently, the buyer would need to purchase both items to use our character properly.
Please add the "default thing" or "default with software" filter where people can see only those items which comes with the software for example default dress, default hair, default eyes etc.
Language is the issue to explain this situation properly but I hope you are getting what exactly I am trying to explain.