Seeking advice on best approach for MMO campaign map

Hi all,

I’m reaching out in the hopes that someone might provide some inspiration as to the best means of creating an MMO campaign map for an WWII idea I’m working on.

The campaign map will serve as a high level of operations for players, while cities and villages on the campaign map will be lobbies created from FPS instances.

I’ve really been scratching my head on this trying to decide how best to create this, while maintaining a level of 3d, much as with Rome Total War, but able to accommodate a large number of concurrent connections.