my list of features needed for a usable audio player:
for audiobooks the ability to skip forward and backwards a few seconds. the best way to do that i found is to swipe left and right. short swipe is a short skip, swipe across the whole screen is maybe a minute. also the ability to quickly jump to a certain manually chosen timestamp.
also for audiobooks, the ability to bookmark positions. not all audio books have chapter markers.
for audiobooks and podcasts: list files by their names in storage sorted by timestamp or name. many podcasts and chapterized audiobooks don't set chapter or episode titles correctly. i have seen players where the title was always the title of the book or the name of the podcast, making it impossible to identify which file was which episode or chapter. also author names and even podcast titles are sometimes inconsistent, because spelling errors, different authors or multiple names for a podcast.
do not assume that the file metadata is consistent or correct and make it easy for the user to work around inconsistencies.
It has 90% of the features that I wanted, and now I am working on what my users want.