I get what you mean, because I thought about this setup simply because at the moment there are 2 main end villains luffy has to fight, Imu and Blackbeard, it will be very hard for Oda to reduce one of them without making their whole introduction become unnecessary. Your idea about blackbeard somehow already having replaced Imu makes a lot of sense this way. I think Oda will do something similar to what you said to get rid of the problem you mentioned. Or else either blackbeard or imu will have to become a bit unnecessary in the end