As an optimization it would be nice if the client could ask for the first 
chunk of data (from the first Pull()) to be returned within 
EnumerateResponse message.

Proposal:

Add a new wsen:Pull element as a child of wsen:Enumerate in the Enumerate 
request:
    <wsen:Enumerate ?>
      <wsen:EndTo>endpoint-reference</wsen:EndTo> ?
      <wsen:Expires>[xs:dateTime | xs:duration]</wsen:Expires> ?
      <wsen:Filter Dialect="xs:anyURI"?> xs:any </wsen:Filter> ?
      <wsen:Pull>
        <wsen:MaxTime>xs:duration</wsen:MaxTime> ?
        <wsen:MaxElements>xs:long</wsen:MaxElements> ?
        <wsen:MaxCharacters>xs:long</wsen:MaxCharacters> ?
      </wsen:Pull> ?
      ?
    </wsen:Enumerate>

/Pull
  basically the same as the Pull operation itself. If the data source
  is unable to satisfy this request then it MAY choose to ignore this
  element.

and add wsen:Items and wsen:EndOfSequence to the EnumerateResponse:
    <wsen:EnumerateResponse ?>
      <wsen:Expires>[xs:dateTime | xs:duration]</wsen:Expires> ?
      <wsen:EnumerationContext>?</wsen:EnumerationContext>
      <wsen:Items>
        <xs:any> enumeration-specific element </xs:any> +
      </wsen:Items> ?
      <wsen:EndOfSequence/> ?
      ?
    </wsen:EnumerateResponse>

/Items
/EndOfSequence
  same as the Pull response stuff.