Spent the last week or so reading the 2012 crossover event that pitted Marvel's two biggest teams against each other, and that heralded a big change in the status quo for the X-Men in particular (I'm assuming, though I haven't read the Avengers books from around that period). I've read that it wasn't universally loved as a story, and I kind of have to agree, since it feels like a number of characters have to act weird for it to work.
Before I get to that, I wanted to consider that statement, that the Avengers and the X-Men are Marvel's two biggest teams. I'm old enough to remember when the X-Men really were Marvel's biggest property, and when nobody really cared about the X-Men, so that feels weird to say. But by then, the Marvel Cinematic Universe had started in earnest, and the crossover came amid the first Avengers movie, which was a big success and seemed to live up to expectations, at least in terms of forming a template for later MCU phases. To put it another way, the Avengers were on the upswing, finally living up to their billing as a top team and pulling in decent sales (I presume).
The X-Men could be said to have been on the opposite trajectory: their most influential writer, Chris Claremont, was long gone from the books (and crucially, from setting the tone editorially) by 2012, despite a couple of returns in the previous decade or so. The two most innovative runs, Grant Morrison's New X-Men and Joss Whedon and John Cassaday's Astonishing X-Men, were over by then too, leaving the mutants in the hands of writers who weren't worse, necessarily, but whose runs don't seem to have captured the imagination of fans as much. I didn't like Peter Milligan's run, for example, while Mike Carey's was good but maybe not as spectacular as I'd have liked. Same with Matt Fraction and Kieron Gillen's respective runs, though both have a lot to recommend them (apart from when the art was done by Greg Land).
The premise of the story felt like it came out of nowhere, a bit: without much foreshadowing, the Phoenix Force is headed toward Earth, and its presumed new host, Hope Summers, the mutant messiah. The Avengers want to stop the Phoenix, given that it eats planets, while the X-Men, against their history, see it as a force of rebirth for the mutant population, which had been cut down to 198 after the House of M crossover. I thought Cyclops's position was particularly strange, since the two previous Phoenix encounters, the early 80s Phoenix Saga and 2004's Planet X storyline, both ended in the death of his girlfriend-then-wife, Jean Grey (she got better in 1986, died in 2004, and got better again in 2018 or so).
On the other hand, given that the Avengers line had expanded to a bunch of new books and teams, it was nice to see how the rest of the Marvel universe reacted to these events. A storyline that I really found intriguing was the Phoenix's ties to Iron Fist's history, since a previous Iron Fist had fought the Phoenix off with the help of Leonardo da Vinci (but of course!). I also quite liked the storyline in Avengers Academy, the book focused on younger heroes, like Wolverine's clone/daughter X-23, in which the younger mutants from the X-Books were held at the Avengers Academy and the two groups debated their own agency. That may have been the most interesting philosophical debate in the entire crossover, given that it hinged on X-23's own experience of being controlled and manipulated by adults.
Not that it was always easy to follow the storyline across the various books. Marvel Unlimited is usually good at gathering these crossovers into reading lists and funneling you through the story, issue by issue, but this crossover was an exception. The reading list put the VS miniseries first, which was focused on individual fights without regard to the overall plot, then the main AVX miniseries came next, and after that the tie-ins that ran in individual series. That would have meant reading the main storyline in the AVX miniseries, then going back to the beginning with the first of the tie-ins, which made it a little more confusing than it should have been. At any rate, there was a lot of going back and forth in time, so it wasn't always clear where a given issue fit in the narrative.
The other thing that came to mind as I read through the crossover was how interesting it would be for one or more MCU phases to focus on this storyline, or something similar. The X-Men are meant to be joining the MCU in the next couple of movies, Doomsday and Secret Wars, which means there could be a big fight between them eventually. I found myself wondering as I read how Marvel might go about it, but it's probably too convoluted and pie-in-the-sky, and would probably be unfair to the X-characters, since they'd have a lot less backstory to make you root for them.
On another note, an MCU version of AVX would probably turn the Scarlet Witch into the villain again, since the whole thing came from her whispering "No more mutants" and depowering them all, which led to this situation. I feel like she's been through enough.
Still, it's a fascinating artifact from the previous decade, when Marvel was on its way to becoming the entertainment juggernaut that we now know it as, and I feel like it has the epic sweep that could be fun as the big crossover movie that caps a few years of individual movies. Though I worry it would turn into a character assassination of Cyclops, featuring him taking on the Phoenix Force and almost destroying the world. He's a favorite character, so I don't like to see him done dirty this way.
In any case, I'm excited to see how the next phase of X-Men comics plays out - I've already read a few of those books, but not in an organized way, so putting them all into context will be like reading them anew. And I'm interested in eventually getting back to AVX via the Avengers books, which should provide a different perspective.
As I said, I don't think it was always fully successful, and it made the X-Men overall look bad, but I have to give Marvel points for ambition. It was developed by all the big Marvel writers of the time, including Brian Michael Bendis and Jonathan Hickman, who went on to be so influential in later X-books, so it's also neat to see their first work on these characters.