The V6 is probably going to be the first thing to be killed off thanks to CAFE. The MPG requirements will be too high for a V6. They will install an I4 in place of the current V6, since it will be able to get closer to the MPG mandate than a V6.
They will need to get great MPG with the majority of the fleet because they will need extra MPG to offset the fuel guzzling ZL1 which will be the only V8 option available. In spite of the factthe ZL1 will be limited production and priced so only a select few will be able to get one, they will still need a lot of I4's to make up for it.