If you’re looking for some type of framework to write the networking portion with, I would be honored if mine was used: https://github.com/baseball435/NetworkingWrapper
It has been pretty popular and I developed it to make it easy to create networked applications. With the library I created a Minecraft mod that allows people to communicate (by chat) across servers and also while in singleplayer. There is a video of it here: Online Chat - Minecraft Forge Mod - YouTube
I also created an Online RPG to show off even more of it’s power: My NEW Online RPG & Networking Library - #1 - YouTube
The framework is very powerful and also very secure and could definitely make cross-server communication easy to implement.