First, I'm not trying to start a philosophical debate...there are so many ways to manage one's tasks and it's very individual. (I'm not a big user of Projects either, for reasons outlined in a recent thread). I'm just talking about what makes the most sense for implementation.
I believe Next Action is a property of a task tree (it doesn't have to be a project), not a property of the grouping or the filter, even though there is a predefined view that uses this property. A next action is a concept from Getting Things Done, defined is the next step that can be taken toward completing a task.
...I just saw your follow-up email and actually that is what I was going to propose. It would be nice in custom filters to allow limiting of results within each grouping (AFTER applying the sort criteria). I would love to be able to do this.
I would not suggest getting rid of Next Actions as a concept as it's pretty key to GTD. Instead, I have commented before that templates should also come with views (and workspaces, now) that support the template structure. In this case I would move the Next Actions *view* into the GTD template. But right now that is not supported.
Lisa