Alternate dimensions/timelines are an important part of one of my settings, a modern fantasy. Magic is well known, and it is possible for objects to be enchanted by a powerful magic user to connect to other universes. Traditionally mirrors are used for this. Each mirror can only be connected to a single alternate universe. These mirrors are very rare and expensive, but luckily one of my main characters happens to own one that he got as a gift.

A major plot point is that while the characters in the main universe, Universe A, can freely go to the other universe, characters in Universe B cannot cross over to Universe A. I have two ideas for the reason:

* It’s illegal in Universe A to bring natives from alternate universes there.
* It’s literally impossible, due to a magical lock keeping people from Universe B from crossing over.

Whatever the reason, this restriction would need to be removed at some point so the plot could progress. Any ideas?

Other Details:

* Universe A is urban, very similar to our world, albeit with several nonhuman sentient species and magic.
* Universe B is a somewhat dystopian version of Universe A, humans live in walled city states, and the other races are forced to live in disorganized settlements
* All my main characterss are not human, so the Universe B main characters live outside these city states.