My advice:
-Plan everything out.
-Make game.
-Charge everyone but me $15.99 per month.
The idea is really just a regular game, but with servers. Other players "send" content through the servers. The servers create enemies, etc. Think about what you want the servers to do, and what you want to be done on each players' machine (if, for instance, the player tries to walk forwards, but you have to tell the server that first and then receive a message from the server in return, it could get slow).
It'll seem a lot easier once you plan everything out.
I know I didn't really answer your question, but I hope that helped.
-Plan everything out.
-Make game.
-Charge everyone but me $15.99 per month.
The idea is really just a regular game, but with servers. Other players "send" content through the servers. The servers create enemies, etc. Think about what you want the servers to do, and what you want to be done on each players' machine (if, for instance, the player tries to walk forwards, but you have to tell the server that first and then receive a message from the server in return, it could get slow).
It'll seem a lot easier once you plan everything out.
I know I didn't really answer your question, but I hope that helped.