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You probably want to issue a secondary hard fork that allows users to send transactions on EthereumClassic/EthereumDark/Titaneum/whatever (hereinafter, "Moose") without those transactions also being valid on the main chain, allowing Moose and Ethereum to actually be separate networks with separate coins without Moose users constantly risking that the transactions they're sending are also affecting their interactions with applications in the Ethereum ecosystem and vice versa. You may also want to give Moose and Ethereum distinctions in the block header format so that light clients can also easily distinguish them. I would recommend the fork just (i) increase the amount by which the transaction nonce in each account increments by 240 instead of 1 after some block N every time a user sends a transaction, and (ii) require the extra data to start with 33 zero bytes (or some similar breaking change). There may be a simpler way to do this but it's worth thinking about.
