I was biased for a long period of time that essential feature of ADC client is NAT-PMP an UPnP support, but SIP programming experience was quite different from my expectations. They don’t care about port forwarding at all! They traverse NAT instead of forwarding ports. And it works like a charm.
I think if ADC is going to move further, Interactive Connection Establishment and WebRTC should be considered as the preferred way of at least file transfers (I can be wrong, but ICE is not suitable for file search).
There are independent STUN and TURN implementations out there, and hub admins can set up multiple STUN & TURN servers.
Another protocol extension can be dedicated to clients advertising STUN & TURN servers. Client software can have STUN & TURN servers built-in, and can help others establish connections. Hubs can check if client is actually functional and give bonuses for those who share resources. E. g. not require share for those ones who provide TURN server.