Great question, Helmut. Thank you for bringing this up.
Abby and the team have been discussing this but we haven't made much progress, and discussing it here is better than talking about it privately.
I have no idea of the answer here.
I'm inclined to think we do need to start establishing a consistent hierarchy for regional and time-period projects, but I don't know what it should be, and I don't know how rigidly we should try to enforce it. So far, as you say, we've been trying to let things bubble up rather than be forced down from the top.
Some random thoughts ...
The members of the Categorization Project have done a fantastic job evolving a solid regional hierarchy. It is complex and duplicative, but it makes sense. For example,
http://www.wikitree.com/wiki/Category:Baden%2C_Deutschland is in the Deutschland and Germany categories and could be in other categories for other names for Germany in other languages. All these are subcategories of names for Europe.
If members came together and wanted to start a Baden Project, it should be a sub-project, but a sub-project of what? Of a Deutschland or a Germany project? Can we have both a Deutschland and a Germany project? (Right now we have a German Roots project.)
I would think that Deutschland would be the right name. Using the native language and current name of a location would be the only universally-consistent rule we could apply. On the other hand, if most of the project members are English-speakers, does this just add unnecessary confusion?
What if instead of focusing on Baden in the 20th century the members were focused on another time period, when it was called something else? If it's Medieval, it might be better to make it a sub-project of a Medieval Europe Project and associate it with the EuroAristo Project since they're mostly Medieval. But maybe they shouldn't have English names.
If European-wide project don't use the English name for "Europe" what do they use? Obviously, Europe is multilingual. Same goes for other top-level regions. Even North America includes Mexico.
The Mexico Project is currently a top-level project, and so is the Latin America Project.
???
Other issues and factors:
Do we really want to force projects to be sub-projects of projects that don't exist? Do we need to create an Oceania Project to encompass the Australia Project? Or does Australia stay a top-level project until there is a New Zealand Project?
How many projects and sub-projects should we have? For technical reasons, we'd like to limit the number of top-level projects. One reason: we can't have an unlimited number of badges.
The current task on the team's to-do list is to come up with better help pages and FAQs on projects. We need better definitions for projects and sub-projects. We have also started thinking of small and/or pending projects in a new way, as a third category of projects we're calling free-space projects. We need to clarify what that means and throw it out to the community here for input.
Drafting these help pages has been on my to-do list for a while. I may get some time this afternoon to work on them.
Chris