... can't it be both?

I feel Twilight's would be the last and most significant. Her element is Magic, after all, aka Friendship. Might be finale material.

But finding the keys seems to be tied to the characters losing their sense of self, and "CMC get popular by hanging with Twilight" doesn't seem to fit what Rarity, Dashie, and Pinkie went through to get their keys :lol

True... plus Twilight has main character powers so with out a doubt be the last of the mane six to receive her key.

Well, it doesn't have to be the finale.
What we presume are the keys have been everyday items given significance. The characters don't know they're keys, they just see them as mementos from new friends.
Twilight getting her key could fall in line, and just receive something like a bell from a friend she made in Appaloosa or something.

All of the "keys" thus far have been items given to the main six from a pony that the inspired their individual element in somewhere done the line. The pony that gives Twilight her's should definitely be one that the audience is familiar with... not saying any names... Trixie.

Well, we got Dash's key from Spitfire in her, what, 6th appearance? Trixie wouldn't be too far fetched. I don't quite understand what they could do since Trixie left Twilight on good terms, though.

But how cute would it be if Twilight's key was Trixie's hat?

Okay, now I want to see that! :lol

I'd be all over that~
http://fc09.deviantart.net/fs70/f/20...rx-d4xhj25.png

Shippers have to be super happy that there's now a cannon moment in which Pinkie Pie is given a big floppy rubber cock.