Thanks for the advice, but although that might perhaps sound like a solution, those project apps that I have seen are not only very complicated, they are also almost impossible to produce a Next list with. They are all based on time planning, which is totally natural because they are intended for hard scheduling across an organization or team. What I am looking for is a simple personal tool.
I believe a source of confusion here might be the terminology. In GTD we talk about projects. That sounds big and organized, but can be very small. And very unpredictable as far as when we will be able to get around to doing it.
Please, let me avoid the common GTD and project terminology, and express my thought in different language:
I want a simple tool, even simpler for me to use than those currently being offered. From a pure user perspective I could describe most of the things I am asking for not as new features but as lifting some very unnecessary restrictions. I do not want to have to learn and combine tons of different features. I want to be able to use a few solid existing features but with less restrictions.
I easily get daunted and blurred and lose perspective when I see long lists, and this hinders me at a practical level. On the one hand I want to break things down to the level where I can "see what doing looks like", and on the other hand I want my lists to be short enough to be reviewable/overviewable/graspable. So what I would like is the ability to simply "group" or "break out" or "collapse" or "relate" things in a way that makes intuitive sense to me and is easy to use. Nothing strange about that, I believe.
Now the easiest way I can think of - for me as a user, already having the two "features" called tasks and projects in my task app - would be for me to be permitted to use those a bit more freely.
If I could drag (or create) a project not only to the list in the left menu (which is a common current restriction), but to any existing project, then I do not have to learn any new features to be able to organize my stuff in such a way that I get both the overview and the detail I want. And, mind you, this is not a "new" feature - just an ability to drag projects around a bit more freely. In technical terms, though, this simple capability (more freedom) is often referred to as subprojects and subsubprojects etc, which sounds way more complicated than it is.
Let me go on. Sometimes I have things (actions, subprojects, whatever) that I could easily classify perfectly correctly as belonging to any one of a number of the larger things on my left menu (projects), because they are fundamental or required for all of these. So I need to decide where to put them. I may be reluctant to put them on the main list (left menu) if that would confuse me - they are perhaps not large or significant enough, and the sheer mass of such items would clutter my left menu, and also they usually do not really relate to the majority of things on the left menu (only to a few), and might therefore confuse me further. I may also be reluctant to put them in any particular one of those projects to which the item could be said to belong - because then I will risk losing sight of this item if that project gets demoted.
Again, the solution is simple. Lifting restrictions; keeping the familiar. I already have a "move" feature, which allows me to drag things (tasks) from one project to another, i.e to have the item indexed from a different project's index. What I am asking for now is a bit more freedom, to let me index my things from more than just one project (index). This would solve the whole problem, and would not require any difficult new features for me in the UI. The only "new feature" required for this is some simple means, such as shift-drag, to distinguish the regular move (copy pointer; delete original pointer) to this "duplicate move" (copy pointer; keep original pointer). In technical terms, this particular capability (to have even more freedom to drag my projects around) would probably be called "multiple parent projects" or something to that tune.
Finally, as for sequential/parallel and all that, let me say that automation is not necessary at all. Handy and clever (and even recommended), yes, but not required. What is fundamentally required is that the app provides some convenient means (out-of-the-box; without workarounds) for the user to select which project tasks (often more than one, but usually not all) are to be visible on the active lists (Next etc) and which ones are to be visible only within the project itself.