Here a simple EA Class diagram collecting Events and 2 specialisations for manual Events and for LogEvents. Additionally there is a 0..n relationship to a list of generic fields (implemented as a simple List.
With Enterprise Architect you can now generate the fitting XSD – Menu|Tools|XML Schema|Generate… – then Generate and View:
Of course you can edit the XSD now and re-import it (of course you loose the properties and methods – the XSD only contains attributes. If you want to do more sophisticated stuff – ask LieberLieber
How would you achieve something like