Take your OTP and write a one-shot about the first and the last time they did something as a couple. Might include “the first and last words they said to each other”, “the first and last time they kissed/had sex”, “the first and last time they saw each other” etc. Bonus: it should be the ACTUAL last time, as in after that, it’ll never happen again, not just “before they fall asleep/until now)