Good news in that there are quite a few side stories to choose from! Any one, or all of them would make for a perfect OVA. Such prequel materials are also from the past to show the characters in a perspective which would be interesting seeing how they react to their world then and now.
Then again even if these do not arrive, the manga especially the anime itself has eternal rewatch value. Even the first two episodes is like a movie in it's own right with beginning, a middle and an end. Same for each 'arc' if you like, all so wondrously sweet!