Automatic port forwarding (UPnP/NAT-PMP/PCP)
To facilitate running servers behind local internet gateways (routers, NAT, etc.), Doomsday should take advantage of UPnP, NAT-PMP and PCP to automatically allow the server's port to be forwarded to the correct place inside the local network.This should be helpful:
#13 Updated by jonesmz about 2 months ago
Another thing to consider is support for the ICE protocol, https://tools.ietf.org/html/rfc8445
This would allow clients to automatically negotiate connection parameters with each other, bypassing the need to use upnp and related tech.
Further, if Doomsday adopts the model of having single player games connecting to a multiplayer server launched automatically, this would allow for clients to dynamically negotiate with each other on which client should act as the server, based on whatever decision making scheme you want.