I like to come back to this thread every once in a while.
Was thinking about some of the plans for future ENT episodes that never were and the idea proposed to set an entire season inside of the Mirror Universe. How could that really be done?
Well, since almost everyone agrees that excising the Temporal Cold War plot elements out of the show is a good idea, something has to fill in the gap for when Archer sacrifices himself at the end of the Xindi arc and wakes up in the Nazi camp. You could simply kill off Archer... but what if he wakes up in the Mirror Universe?
Ignore "In A Mirror, Darkly" for a moment. Archer's counterpart would have died in some similar event to what occurs in the main universe (the conquest of the Xindi, no doubt) and Archer would assume his identity, allowing all of the main cast to remain on board in warped roles while Archer's characterization remains the same--and that would be the part most disturbing to him, is that he fits so well into the Mirror Universe. We've already seen Archer at some pretty significant low points during the Xindi arc and he has what seems like severe trauma from it all.
Archer would spend a season (or maybe just half a season) in the Mirror Universe, slowly being undone by it. This would be his "A Christmas Carol" moment, realizing what kind of path his actions and Humanity as a whole could go down. Eventually an event similar to that in "In A Mirror, Darkly" occurs where the Tholians have opened up an interdimensional rift, but this one doesn't connect back to the TOS episode. Archer makes his escape through it and returns to the Prime Universe.
Now, this might look like it violates the events of "Mirror, Mirror" as being first contact with the Mirror Universe, but by making Archer the only visitor to the Mirror Universe it 1) makes his story suspect, and 2) since he arrived by virtue of a near-death experience, he might have thought it was all made up in his head anyway, or evidence of an afterlife (a personal Hell, probably). The people in the Mirror Universe would also continue to assume that Prime-Archer is Mirror-Archer.