Right now, only the file name supports being either a string, symbol or function, etc. Nonetheless, it would be nice if headline could be a function that takes in as parameter a list of all headlines present in the file so that we can dynamically pick a headline under which we want to add an entry. The function can also return a string to represent a (potentially) new headline to insert.
My use case for that is I have an org file in which each headline is a specific time, and I want to reuse a headline that is only within 5 minutes behind the current time.
