I don't believe this has already been asked, but I might be wrong; sorry. One can add a magnetic charge density/magnetic monopoles to Maxwell's equations to make the theory symmetric between the electric and magnetic fields. What happens when we quantise this theory?
Since QED is a $U(1)$ gauge theory, would we expect this theory to be a $U(1) \times U(1)$ gauge theory? This seems a bit naive, but it makes some sense since you have two charges instead of one, acting in the same way (possibly different coupling constants), hence two gauge groups instead of one. This gauge theory is still abelian, so it seems similar enough to QED.
I wonder about how the electron and monopole interact in this theory, and if that needs to be added in some way, and if perhaps we have too much additional symmetry, since QED already contains magnetism - it just lacks sources.