I tend to keep actions I can't do yet attached to a project, but with a different todo keyword (this is in emacs org-mode) such as WAITING or NEEDSPREREQ. Normally these actions get added to a project during Inbox processing. When I mark an action as DONE, I do a mini-review of that project to see if any non-doable actions have become doable, in which case they get marked as NEXTACTION. New actions that I think of, doable or not, get thrown in the Inbox (with org-remember).

I also keep all independent actions that move a project forward marked as NEXTACTION, not just one. In my opinion, that would just be an arbitrary restriction that's not related to the primary feature of Next Actions, which is that they are concrete things that can be done immediately in the right context.