Perhaps the Lobby plugin should provide an empty arena object. This defines the basic items a plugin can have, for example a list of players that are inside the arena. Then a different plugin can expand upon that by overriding the functions. I did the same for my Catastrophes plugin. There is an object that defines the most basic things like the position, speed and world, and other catastrophes expand upon that. They can override functions that the basic object provided.
In this case plugin's should provide a file that the lobby plugin would load. If an arena needs specific data from the plugin it's created from it can simply use CallPlugin to call an API function there.
In this case plugin's should provide a file that the lobby plugin would load. If an arena needs specific data from the plugin it's created from it can simply use CallPlugin to call an API function there.