Building a map together can be fun, but it would much better to have a competitive element to the multiplayer aspect.
Depending on the code structure, I believe it shouldn't be too hard to implement.
Multiple parks in the same map. Each player will have its own park to manage (possibly each park can be managed by multiple players, requiring consent from the other players of that park).
Park guests should choose one of the parks to enter, based on park rating, pricing, guest preferences and so on.
Each player is restricted to build and manage their own park.
Finances, guest ratings etc. for all parks should be visible to all players.
Let me know what you think. I might be able to help with the implementation, although I am not that experienced in C++.