I wish something like this existed that could additionally proxy FCM notifications. Allowing push notifications without any Google involvement is the ideal long term goal, but one transitional path I dream about would be forcing Google to proxy all their notifications through a host of your choice (ideally that you control).

I think I have about 30 false start projects with names like PushMux and rough outlines on potential ways to accomplish it.

Tangentially, I also dream of having true one-way push notifications. Send a minimal notification to the target device via FLEX/POCSAG or even one-way Iridium, and allow the device to connect to the internet as needed to fetch the rest of the notification.