The fork itself won't have any impact on BTC's market cap; at least, it didn't with the last one in July. The number of BTC coins is not going to to change. An equivalent number of BTCGold coins will be added, which will have its own market cap based on whatever value the market determines for each coin.
Again, I'm hoping it stays that way but the fact that they're wanting to use the same UUID for the transactions make me wonder. Anyways, it's still speculation at this point since no one has solidified on what it is that they are doing.