Mark Ruffalo has no idea whether or not the Hulk will ever return to the Marvel Cinematic Universe. Ruffalo has played Bruce Banner (and his big green alter ego) since the MCU's first team-up in 2012, The Avengers. Since then, he has been featured prominently in each of the Avengers movies, guest-starred in Thor: Ragnarok, and made a mid-credits cameo in Iron Man 3. Despite this recurring role in the larger series, Ruffalo's Hulk has never been given a solo movie of his own.
The Hulk's history on the big screen is a curious one. After a less-than-loved adaptation by Ang Lee in 2003, the character headlined the second-ever MCU movie, 2008's The Incredible Hulk. That film starred Edward Norton in the role, but he was replaced with Ruffalo after the film's release, a recasting that remains somewhat mysterious. Ever since then, Ruffalo's Hulk has been on quite the journey. After struggling to find a balance between his two halves in The Avengers and Age of Ultron, Hulk disappeared until Ragnarok, where it was revealed he had been 100% Hulk for years. After that, the Hulk took on Thanos and then went MIA, before reconciling with Banner in the merged personality of Professor Hulk, as seen in Avengers: Endgame.
During an interview on The Graham Norton Show (per comicbook.com), Ruffalo confessed to being in the dark about where the Hulk goes from here. When asked if he would be returning to the franchise, he said, "I don't know. Who knows, really, it is supposed to be over, and that was supposed to be the end." The "that" here refers to Avengers: Endgame, in which Black Widow and Iron Man's heroic sacrifices saved the universe, and Captain America gave up his shield to live out his life with Peggy Carter. While many characters in the film were given definitive endings, Hulk, on the other hand, is still very much a part of the ongoing MCU.
Barring another surprise appearance, a la Ragnarok, there doesn't seem to be any immediate plans to bring Hulk back into the fold. Marvel has announced a full slate of films and TV shows well into 2022 that includes sequels for characters like Thor, Black Panther, and Captain Marvel, new franchise starters like The Eternals and Blade, and even a long-overdue solo movie for Black Widow. The only place that currently makes sense for Bruce Banner to return is the upcoming She-Hulk show, but as of now, no plot details are known.
But the biggest unknown at the moment is how the MCU will handle its future team-ups, a decision that will likely be the deciding factor in Hulk's potential return. There's currently no story reason to continue Banner and Hulk's journey, and with all his closest friends either dead, retired, or off exploring the cosmos, there's not much reason for anyone to call him in. What the Hulk needs then, is the next big threat where the new Avengers have no choice but to bring him back, but who knows how long it will take the MCU to build up to that.
Source: The Graham Norton Show (via comicbook.com)