The country profile would contain a link to the 'Regions' rankings, just like it does with Cities; every Region would have its own profile page, modelled in the same way as those of Countries. Links to the Region profile would also have to be provided in every City profile (especially because, from a technical point of view, I guess the handiest way to make regional statistics possible would just be to manually 'associate' every city to its region).
Regions of different countries could be added at different times, because they just affect pages related to that country, and no 'general' ranking featuring regions from different countries would be needed, because the sizes of regions can vary according to the different countries, and then not every country is suitable for being divided into regions. Actually most of the countries will never need this division (think of small Eurozone states, but also to all non-Euro countries, where the number of notes is very low).
This would allow to start from the countries which are most in need (De?, or even Finland?), or from those where it is simpler to differentiate towns into regions (for example, in Italy it can be done automatically according to the post code).
I guess that would be a VERY hard work, but, well, I came up with this a few days ago while comparing hit ratios from different Italian towns and probably this feature would prove very interesting
Be sure that the outcome would be extremely interesting. Please see that this work cannot be achieved by one person only. And the data had to be collected which town belongs to which region / county in a country. 