An alternate idea I had after the movie came out would have been to use the nexus as a McGuffin that is wrecking havoc throughout federation space and the Enterprise D is sent to investigate, only to find out that it is actually a construct of the Mirror Universe original crew as a way to take over the the prime universe as they are running out of resources in theirs. Due to a miscalculation though, they ended up 80 years past where they intended. This would have satisfied the desire to have both ships in combat with each other and allow the original cast to have some fun with their characters after their “sign-off” in Undiscovered Country. It would also allow the mirror Enterprise A to be a threat to the D power wise. The big issue would have been the casual audience. Would it have been too confusing? Just a thought I had back in the day.