I haven't thought about this subject yet, but for MN, a tag line might be "There is a right & a wrong way to do everything". 

I imagine for The Magician's Nephew, it'll be something like "Every world has a beginning",or something similar.

It'd be really cool if the tagline were a line from the books but I'm not sure if it'd be practical. A tagline should probably be something that sticks in everyone's head and is somewhat understandable to everyone, not just people already familiar with the story. Like if I saw "There is the utter East" on a poster for a Voyage of the Dawn Treader movie or "Remember the signs" on a poster for a Silver Chair movie, I'd be delighted but non-fans might think it just sounded weird and confusing and forget about it. 

However, since people generally know what Narnia is even if they haven't read any of the books, they could get away with "Narnia awake" as a tagline for The Magician's Nephew and since The Lion, the Witch and the Wardrobe is the most famous Narnia story, they might get away with "Aslan is on the movie" as a tagline for that movie.

By that logic, they just might be able to use "Narnia and the North" as a tagline for a Horse and his Boy adaptation though the North part might seem a little random. 

My first thought "The end of the World" Actually that could be used for 3 books.

MN Charn ends

VDT: They sail to the end of the world

LB: The end of Narnia

MN: The wood before the Wardrobe 

MN - Narnia Begins
LWW - Discover Narnia Again
PC - Narnia Calls
VDT - Sail the Seas of Narnia
SC - The Wider World of Narnia
HHB - A Narnian Adventure
LB - The End of the Beginning

I think "Narnia Awakes" is the best idea so far for. MN, since of course it reflects how Aslan calls the Talking Beasts and other sentient beings at the climax of the creation scene.

It's difficult, though, to think of tag lines that are memorable and meaningful without sounding clichéd. I'll be interested to see what Netflix comes up with for this and (hopefully) subsequent adaptations.

One thing I just thought of is that movie trailers very often have a sort of extended tag line, either spoken by the voice-over or shown in writing, bit by bit at dramatic intervals in between clips from the film. I'm sure movie buffs here will know what I mean (there's probably a technical term for it). Any ideas for how that could be done with a Narnia trailer? 

Narnia comes alive, might be even better as a tag line for MN, not just waking up?

For HHB, a good tagline, in my view, remains Bree's mantra: Narnia & the North.

For VDT, what about To the Utter East? Reepicheep's mantra, this time.

For SC, Follow the signs is a must, in my opinion. By the way, I still have the Narnia Web T-shirt with that motto on it.

For LB, All good things come to an end? 

That leaves PC and LWW, to think about. PC already has an alternative title called "Return to Narnia".  For LWW, what about: A most incredible adventure? 

Some suggestions for The Magician's Nephew (based on quotes / paraphrases from the book):

'The beginning of the comings and goings between our world and the Narnia'
“Wouldn’t you like a ring, my dear?”
"You might meet anything — anything.”
'Are you ready to undo the wrong that you have done to Narnia on the day of its birth?'