A possible plot is that they start a secret project while in hiding. Maybe they find an old, broken radio and try to fix it together in secret, not just as a way to pass the time but also as a symbol of hope, a connection to the outside world. This could bring them closer and also cause some tension as they try to keep it hidden from the others. And they could have a special code related to the radio that only they understand.
One idea could be exploring a different scenario of their relationship. For example, what if they had more time to bond before being discovered. They could have shared more of their dreams and fears, and perhaps Peter could have taught Anne more about the outside world based on his memories. Another plot could be a post - war scenario where they reunite and have to deal with the trauma they've endured together.