Collectively, House of X and Powers of X changed the world of the X-Men more than any other story has in years. After those stories established the new mutant nation Krakoa and made the survival of mutants more uncertain than ever, the subsequent "Dawn of X" relaunch gave all of Marvel's a cohesive throughline.
However, one of the biggest ideas that House of X established is still missing. While Moira MacTaggert was the catalyst for all of the changes we've seen, she has remained almost entirely absent from the X-Men's world since those series ended.
In Powers of X, it is revealed that Doctor Moira MacTaggert, the X-Men's longtime ally, was, in fact, a mutant with the power of reincarnation through which she would go back to the moment of her conception with all of the memories of her previous lives. This Groundhog Day-esque mutant ability allowed her the chance to see how various different outcomes and futures played out for mutantkind.
Through these lives, Moira learned that the only way for the mutant race to survive was through the creation of a haven like Krakoa and through destroying the Sentinel technology that led to the creation of the mutant-hunting Nimrod on the Orchis Forge space station.
While that mission wasn't as successful as it initially seemed, the X-Men have turned their attention to the various other threats that are sitting in front of mutantkind. Kate Pryde and the Marauders are sailing around the world, X-Force is fighting evil plants, and Excalibur is dealing with Otherworld.
However, Moira MacTaggert hasn't been present for any of this. Instead, she's remained hidden within the No Space in Krakoa, where her very existence is a mystery to most of Marvel's mutants.
Moira's last actual appearance was in Jonathan Hickman, R.B. Silva and Pepe Larraz's Powers of X #6, where she met with Magneto and Professor X in her house in the No Space. While there, she begins to explain the reasoning behind her secrecy. To reveal her mutant status would be to reveal the terrible knowledge which she has accrued throughout her many lives, and the knowledge of how doomed mutantkind really is could be ruinous for Krakoa. This is also the reason that she instated a ban on precognitive mutants like Destiny who Mystique has specifically asked to be resurrected.
Even though Moira is one of Krakoa's founders, it seems strange for her to simply step aside completely from the fledgling nation-state, even though this is what she tells Professor X and Magneto she is going to do.
While Moira's absence until now is curious, it's almost certainly only a matter of time until she returns to the X-Men's world.
Although House of X and Powers of X revealed the details of many of her past lives, there are still a lot of gaps in her history, and the events of her sixth life are still completely unknown. Late last year, Hickman teased an upcoming Moira solo series, which has still not yet been announced or officially materialized. It's almost certainly a matter of if -- not when -- Moira will return, and she could very well make a reappearance in the upcoming X of Swords crossover.