Synchronization between two servers

I have the feeling that your many  "cross server " requests will go unrequited

One challenge with synchronizing data between servers is that different sever networks may want to use different designs. The logic needed would likely also be very similar across plugins, so ideally most of it could be shared in a generic way, and/or some learnings could be applied such as to implement standard hooks and API methods into most plugins to allow getting at the important underlying data. Somebody will need to come up with a technical design that makes sense.

For now, one option is that you can implement your own plugin to copy data files between servers on player disconnect. The only issue you will run into is that the player's backpack data file may not be written yet, so some data is only available in memory. A short term solution to dealing with that problem is to set the config option for saving data only on server save to false, so that the backpack will be saved as soon as it is closed.