It would be nice in addition to Raw Only fields to be able to create Transformation Only fields, which as the name suggests would only exist in the Transformation view.
In our current setup we generate a business key based on a combination of the DW_SourceCode and the code in the source system. This is then used to generate a Supernatural Key. These Business key fields are set on Raw Only but as they do not yet exist in the raw table, they are empty nor do they exist in the Valid table since they are Raw only.
It makes no sense to have all these empty Business key columns in the Raw tables and an option to make these Transformation Only would be a nice addition.