Hi,
In GTD you plan your project by defining tasks. Not all of these tasks can be done at the beginning. So you mark the ones that can be started at that time as Next Actions. As you go on an do those actions, some other tasks that you have planned before become next actions. So, It would be necessary in a GTD implementation to be able to define task that are not in Next Action, Somday, etc. In the reviewing process you should be able to put the in any of these boxes. This can be achieved by letting the focus box of tasks empty.
Although by defining task dependence, only relevant tasks would appear in Next Action list, but defining dependence among task is a sophisticated and time consuming process that should be optional.
So I suggest to let define tasks in projects without any focus box and put them in boxes such as Next Action as you go through the project. Otherwise your next actions list will become large and contain tasks that are not next action or you have to define task dependencies (if implemented) which is time consuming and not a mandatory GTD concept.

regards,

Parham