I haven't played one of these in a while...
The biggest thing I'm interested in with this game is the educational element, where beginners can learn about flags and hone their skills. Now, I've always been really bad at flags and geography in general, so I wanted to see how far I could get with learning them.
I couldn't stay engaged for long with the learning process, probably because of how bland it felt to me. I would click a flag I barely know to a country I've never heard before, get it wrong (or right by dumb luck), and move on. I didn't feel like I was learning or improving very much...
Something that was working out with this system was this: Sometimes the same flag from a question ago would appear again, and I would get it right, because I remembered what it was. I recommend doing this more, and more often, a it uses short-term memory to aid in learning. Maybe you can put this in a separate learning mode, if you would rather keep the beginner mode you have.