Some apps that I don't understand why no OSS exists:
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Teleprompter app that allows you to read a scrolling script while recording video
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basic photo editor to crop, rotate, color correct, add text
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basic video editor to crop, clip, and combine video
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visual voicemail
And just for fun, here are some OSS apps that are better than any non-free alternative: SD Maid, Firefox/Fennec, Aurora Store (OSS front-end for a very proprietary Google store), RTranslator, Syncthing, OSS Document Scanner.