Warning: Contains spoilers for Avengers #57
The Avengers are supposed to be Marvel Comics' flagship superhero team, the publisher's equivalent of DC's Justice League, but they have been sidelined due to the increasing prevalence of the Multiverse in stories, to the point that the group does not even appear in the latest issue of their own comic book.
The Avengers began as a simple but effective idea: having some of Marvel's already-existing characters team up in shared adventures. This idea of an "ensemble cast" of heroes is very different from, for example, the X-Men or the Fantastic Four, where the characters are created specifically to be part of that team, and the group identity is central. Over the years, the Avengers grew to be Marvel's most popular team, despite their rotating roster, and the whole concept of "the Earth's Mightiest Heroes" became central to their identity. The Avengers are supposed to be the best that Earth has to offer, and for this reason, they are always on the frontlines during Marvel's big events.
Despite the Avengers' enduring popularity, however, they have been sidelined in their current ongoing series, which has been focusing on the concept of the Multiverse. The demon lord Mephisto has assembled a terrifying team of multi-dimensional villains called the Multiversal Masters of Evil, and they are conquering one parallel Earth after another. For this reason, many issues in the series have been devoted to showing these alternate universes. Avengers #57, by Jason Aaron, Javier Garròn, and David Curiel, focuses on Sebastian Szardos, the Soldier Supreme, a unique mash-up of WWII Captain America and Doctor Strange, and his battle against Mephisto in 1943. The Avengers don't even appear in the issue, besides in the background of a few panels while they covertly help Szardos win his battle.
The Avengers' Multiverse plot is getting so big that it's hard to track what is going on. It's unclear how the Avengers got to Szardos' universe, when they decided to travel in the Multiverse, and for what reason. It is hinted that they decided to stop Mephisto by gathering "history's mightiest heroes," a roster of alternate versions of the Avengers. Nighthawk said in issue #55 that he needed a "time machine" for his plan to trigger a confrontation with Mephisto, but all that doesn't change the fact that the actual Avengers almost do not appear in their own series. The Soldier Supreme is an interesting character, but he is unlikely to become one of Marvel's flagship heroes like the Avengers are supposed to be. With so many variants of worlds and heroes being constantly introduced, the risk is that the "originals" get sidelined. The problem is evident from the cover of Avengers #57, where there are so many new characters that it is hard to tell who the actual Avengers are.
The concept of a Multiverse is all the rage in comics right now, and it offers very interesting possibilities for creators to debut new characters. However, focusing too much on expanding the Multiverse and introducing a plethora of new worlds and heroes carries the risk of shifting too much of the focus away from the heroes who have been at the cornerstone of Marvel for decades. The Avengers should still be Marvel's most important characters, at least in their own series.