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You did not address the support for sub-categories issue. wp_wp_eStore_cat_tbl contains cat_parent and this shows you intended to do this; and, you think it is important. Because this capability is included in your schema there will be no database migration issues for existing users. Any store can benefit from sub-categories. Large stores need it. Your schema supports nesting sub-categories to any depth. I recognize that code to support this is not trivial; but, others have most certainly done it so examples must exist. Short-codes to display products which have membership in two or more categories would complete the picture. What do you think? Is this in eStores future?