A camera app on your phone, that can't access your files, and has no inbuilt filters. You take the photo, and it instantly gets uploaded to the photo sharing service. No editing, no offline photo manipulation. What you shoot, is what what you share. Deleting a previously uploaded photo should be possible, but a deliberately complex affair, to discourage image curating.
I doubt anyone would use this kind of app, but it's nice to imagine something like it existing.
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[1] Instagram was originally 'as-shot + filters only' on your phone. Later, they caved and allowed previously shot photos on your camera roll to be uploaded, which led to an abundance of phone photo editing apps, then finally they allowed desktop uploads, and that was the end of what made Instagram different.