Fixing the Serana wait glitch via the console
 

If Serenas dialog "Come with me" is missing when she's waiting somewhere -
here's the fix:

Unlike followers from the vanilla game, Serana's dialogue does not check the normal follower quest, but certain quest variables in the quest "DLC1NPCMentalModel".

You can take a look at your Quest variables with the console command "sqv DLC1NPCMentalModel".

The following conditions must be met, for her to be available as a follower: CanFollow_var = True / IsFollowing_var = False / IsWaiting_var = True / TurnOffComeWithMe_var = False / LockedIn_var = False

In my case the glitch was triggered by the LockedIn_var being set to True.

You can change the variable by typing "setpqv DLC1NPCMentalModel LockedIn_var False".

You should check all 5 variables in your game, by scrolling through the console with the Up- and Down-Arrow keys.

You're like the only one on the internet who'd posted a valid fix for Serana's delicate "issue", I've searched thoroughly.
I decided to try and set her dismissed state to true since she was neither dismissed nor following which didn't make sense, and setting her to waiting didn't help but setting her to dismissed then asking her to follow finally worked. Many many thanks! :)

Thanks for the info I had the same happen to a few other companions in the game also.