Indeed it will be very interesting to see how products evolve over time. Open Food Facts also has a "Food Archeology" project. The idea is to find old products and put them in the database as well. Anyone knows soda cans collectors? :-)
Something very interesting as well is to compare products accross different countries, first to identify differences, and second to see if there is a correlation between those differences and differences in the prevalence of some affections like diabetes or obesity.
It might be difficult to track the products which change labels as ingredients change.
Another interesting idea would be to see how product lines evolve. Watch as the product line is split into two or more lines to target different types of consumers (or merged for that matter), while at the same time keeping record of ingredients and other interesting info. Sounds fun!
Something very interesting as well is to compare products accross different countries, first to identify differences, and second to see if there is a correlation between those differences and differences in the prevalence of some affections like diabetes or obesity.