I think it would be cool if smartphones had a paradigm of user-controlled back-end server. All the push notification registration and compute-intensive or network-intensive operations could run on a home server that a phone is paired to. I already choose apps with an option for a self-hosted backend where possible, but its not the default. And I can't run my own Firebase Cloud Messaging server or my own APNs at all.
A lot of smartphone users have IoT devices plugged in at home that, for better or for worse, have a persistent connection to the internet. It would be feasible and useful for these devices to act as personal servers for certain kinds of services. In fact I believe Apple already does this with HomeKit, where users’ AppleTVs or HomePods can act as a “hub” to facilitate communication with devices at home from outside the local network.
