Auto-download subtitles
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The feature I miss the most is auto-downloading of subtitles. You can make this premium-only feature and I would buy a lifetime licence immediately.
OpenSubtitles org have an API which you can use. See at BSPlayer how they do it, they pull subtitles from various sources and even upload subtitles to their database to be better next time. I believe it can be done much better. The only reason I use BSPlayer is this feature. Otherwise it's a horrible player with awful UX. But I'm just too lazy to look up subtitles manually.