The Russo Brothers are only interested in returning to the Marvel Cinematic Universe after Avengers: Infinity War, they're prepared to step away after Avengers 4.
Anthony and Joe got their start with Marvel Studios by directing Captain America: Civil War, which essentially became a testing ground to see what it'd be like to direct an Avengers movie. They've completed work on one and released it, but are still in the post-production process on Avengers 4. Despite that movie not even having a title or a trailer, fans aren't ready to see the Russos leave the MCU behind.
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Following an Avengers: Infinity War screening hosted by Collider, the Russos were asked whether or not they were really going to step away after Avengers 4. Sideshow Collectibles was present for the screening and the Q&A that followed and shared their response. It appears they're only interested in doing Secret Wars at this point. When asked if they'd return to the MCU, Joe Russo said, "Not until they’re ready to make Secret Wars."
This is not the first time the Russos have expressed interest in tackling a Secret Wars adaptation in the MCU either. Their interest was previously made known after Disney began the process to acquire Fox. The acquisition will see Marvel Studios gain access to the X-Men and Fantastic Four, and the possibilities with the storyline and the volume of characters is what the Russos appear to be particularly interested in. The original Secret Wars was a Marvel event from 1984, but the comic company recently put a new spin on the title in 2015 as a means to destroy the multiverse.
Even though the Russos may currently have their eyes set on Secret Wars, the chances of that happening are not clear. They've remained could be their best work yet, allowing them to go out on an extremely high note.
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Source: Sideshow Collectibles