I have a very similar question.
I would like to have an option for the user to select a type of content (e.g. Html Content, Content Include, User Poll, etc), and then be presented with a set of fields specific to their selection. This way, they could order the pieces of content however they'd like, and not be stuck entering them in a rigid sequence.
The Wiki Forms Authoring Guide says to use an xf:switch in place of an xs:choice. It mentions in the Supported XML Schema Data Types that xs:complexContent creates an xf:switch, but doesn't give anymore detail…so I'm stuck.