The WBS is a representation of the work that needs to be undertaken to produce the deliverables identified in the Phase Zero report. It is likely to go through a couple of iterations as you put it together. You should seek input from people involved in the project rather than rely on your own input. If the Sponsor is available, it is useful to have him or her provide their thoughts on the work.
Although it is yet to be addressed, look at what quality activities may be included and be sure to build them into the WBS. Also be sure to allow time for rework after a quality review.