This one will be a hella sad book to write.
It's an AU where you know how many more days/years left you have to live.
Most oftenly the MC will either have an infinite life time— which, in my opinion, will be a thrilling adventure for sure— or only a few days/weeks left, which can get very sad.
Idea:
I saw one like this where the days you have to live is chosen on cards that you take at a gambling centre or something— and the person got infinity.
