There's a new initiative called 'eelo'[1] which claims that it will be a drop-in ROM replacement for existing Android phones. Apparently it's built using a modified AOSP[2] using their own open-source software components. They are focusing on privacy and security. I'm not convinced they'll ever release anything substantial, but it's good to see another player in the Mobile OS space trying to challenge the status quo.
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[1] https://www.eelo.io/mobile-phones/
[2] https://source.android.com/