"The Key" is very likely going to be a big piece of whatever is going to come next... As it had the the Sun symbol on it when it was activated by the Everqueen/Queen Eternal, that particular Sun symbol being Azem's astrological sign.
We also learned that it came to Alexandria from the Source during The Sixth Umbral Calamity, a.k.a. the great flood as it was used to open a portal by Lalafels on the Aloalo Islands to escape it.

The people over at whatever reflection Alexandria was in, were able to use The Key to open a portal to the Source. While we've only received confirmation it works between those two reflections, it doesn't mean it won't work on others.
And because of potential travel between reflections, I foresee Y'Shtola being present due to her interest in the subject.