I2p can't really work well on a per app integration basis because it requires uptime to be able to make connections. It doesn't have a centralized database of all of the nodes like Tor has. It has to discover them. We're unlikely to see per app integration for this reason.
Really though, apps allowing routing traffic via SOCKS for both Tor and I2P is more ideal than every app having their own Tor daemon. This way you can ensure Tor is updated and has appropriate settings, you don't have to trust each apps implementation.
I've just wrote one such mechanism myself.
Relays are setup by volunteers on virtual machines. Devices from users choose which relays to be reachable from (based on region or availability) and relays share the list of connected devices between themselves.
This way phones can connect quasi-P2P between them with a very light format that doesn't waste battery and doesn't require "clouds" for storage.
Very similar to NOSTR relays, the main difference are socks and a few extra services like file sharing and search.
