If you want to send message from and to arbitrary nodes of your network, every node must have a unique address. Then you can connect all nodes to a singe I2C, Ethernet or any other bus.
Otherwise you need hardware that can listen to a number of channels at the same time, like the four Serial channels of a Mega. Or each node must supply signals indicating ready-to-send/receive, so that a next-level controller can serve the individual channels.