The most iconic, innovative, and overall awesome crossover between Marvel and DC was the Amalgam Universe, as it not only brought the two separate worlds together, but also merged a number of their respective characters from such teams as the X-Men, Avengers, and Justice League. However, there was one character-a mutant-who was effectively neutralized within this crossover, as she was simply too powerful for this new reality.
Moira MacTaggert was revealed to be a mutant with the power of reincarnation in the era-defining House of X/Powers of X event. Not only does Moira get another chance at life after every one of her deaths (or, at least ten to eleven deaths, anyway), but she restarts the entire universe every time she's reborn. Plus, Moira keeps all her memories from the life before in her new life, which allows her the potential to reshape the new world however she can. This is how the mutant nation of Krakoa was born, and how fans learned that the ultimate villains of the X-Men are and have always been the Sentinels. However, beyond in-canon pieces of information derived from Moira's mutation, her powers alone make her one of, if not the most powerful mutant in the Marvel Universe-and two cosmic gods must have known that.
In X-Patrol #1 by Barbara Kesel, Karl Kesel, and Roger Cruz, readers are introduced to the amalgamation of the X-Men and Doom Patrol (and kind of the Teen Titans, too) with X-Patrol. Their mission is to defeat perhaps the strongest Doctor Doom variant in existence, Doctor Doomsday, but not before they all spend some time training and getting to know each other better. This issue actually marks the formation of the X-Patrol, so these heroes needed to learn how to fight together before being plunged into battle against a cosmic-level threat. During their meet-and-greet, Beastling (the amalgamation of Beast and Beast Boy) makes a seemingly innocuous comment about something reminding him of his first grade teacher, Mrs. MacTaggert.
This issue seems to confirm that Moira MacTaggert is a first grade teacher within the Amalgam Universe as opposed to the mutant geneticist and long-time friend and ally of the X-Men that she originally was in Earth-616. While this may have been a bit surprising upon this comic's release in the mid-'90s, it makes perfect sense as to why this was the case today. The Amalgam Universe was created by two cosmic gods, each one watching over the Marvel and DC universes, respectively. As it turned out, these gods weren't supposed to be two, they were always meant to be one. So, when they discovered each other's existence, they merged, and with them, their universes merged as well. These gods had the chance to start from scratch and create a new universe that matched their newfound form, so it makes sense that they'd want to neutralize anything or anyone with the power to restart that universe without their approval.
Moira MacTaggert has always had the power to restart the universe with her deaths, fans just didn't know about it until House of X/Powers of X. But, these cosmic gods seemingly knew about all the way back in the '90s, which is why they neutralized her by cutting her off from the main-players of the Marvel and DC Universes. Being a teacher is a noble profession, but teaching first grade is a far cry from starting an entire mutant nation with the most powerful people on the planet. Yet, this was the fate of X-Men's Moira MacTaggert in the Amalgam universe, as she was seemingly deemed too powerful for this Marvel/DC crossover.