To bring back the "Come with me" dialogue after dismissing Serana, enter the code sqv DLC1NPCMentalModel in the console. You are given a long list of information. Scroll down the list using the up and down arrows, and make sure that the following codes: CanFollow_var = True / IsFollowing_var = True / IsWaiting_var = True / TurnOffComeWithMe_var = False / LockedIn_var = False are in order by looking at the false/true (i.e., if LockedIn_var was set to true then you need to put it to false). To do this, enter the following command: setpqv DLC1NPCMentalModel LockedIn_var False. Once you do this you should be able to recruit Serana to be your follower again.