What I'd like to have is easy offline media files sharing across as many phone models as possible. Basically a free open-source alternative to ShareIt. Why it doesn't exist yet? Does ShareIt use some hidden and/or proprietary APIs?
I do this with the Manyverse app, which uses the Secure Scuttlebutt p2p gossip protocol.
if "on the local lan" is offline enough for you, take a look at kde connect, connects all your devices with file transfer, cross-device copy'n'paste etc.
By "offline" I mean not using any external infrastructure at all. Use Wi-Fi/bluetooth/whatever both devices are capable of and set it up as needed. I want to walk up to a person anywhere and share photos/videos I've just made with them.