This is going to be in flash, correct? If so, is entire game contained into the SWF file, or is it just a flash client accessing a server to run? Does the game store player data on a server, online database file, or on a database in your own computer? If it stores data on your computer and the entire game is contained in the flash file, is it peer-to-peer, or will it need to access a server?
If the game is contained in an SWF file, contains all player data on the person's computer rather than a server, and is peer-to-peer, and offline mode would not be that difficult to implement. Just disable the script that attempts to connect the game to other players. Players would need to download the SWF file, though.
Sorry if you don't understand these questions, or don't know the answer to them. By the way, the menu looks great.