I'm currently learning on how to integrate this to create notifications that don't use the google servers for my android phone. It seems very interesting.
https://unifiedpush.org/
UnifiedPush is a set of specifications and tools that lets the user choose how push notifications* are delivered. All in a free and open source way.
This is a list of apps that currently use it.
https://unifiedpush.org/users/apps/
here are the connector libraries for our apps
https://unifiedpush.org/developers/
I also wonder, if SN could use this so I can enable notifications on my phone!