OIC. In that case, you may want to do a search for XML Schema repositories. You might find one or more of the schemas at sites like this fit your requirements.

In the big picture though, such schemas are either wholly inadequate because they do not cover everything you need; or wholly unmanageable because they do.

Whilst they might serve you as a starting point, as soon as you reject one of their included attributes; or add a new one of your own, you've effectively made it up as you've gone anyway and any probative value that using it as a starting point might have had is lost.