This already exists in the form of Look and Feel packs. It may not have support for all of those settings right now, but it would be better to extend Look and Feel than create a new type of theme pack.
I personally feel like the most intuitive way is to create a KCM just for Layout, and then make the Look and Feel able to override those setting. In this way if you only want to change the layout you will change it in the Layout KCM, but Look and Feel would still be able to change the layout. If I recall correctly, that should be the same relationship between Look and Feel and Plasma Theme.