Warning: SPOILERS Below for Avengers: Infinity War!
After Avengers: Infinity War, the Avengers' biggest problem is still Thanos, but there's an old issue hanging over them: their violation of the Sokovia Accords. The legislation requiring all 'enhanced' individuals and operatives of the Avengers to comply with UN oversight was introduced in Captain America: Civil War. It was the story point that divided Earth's Mightiest Heroes and ultimately forced Captain America, Black Widow, the Falcon, and Scarlet Witch on the run for two years as wanted fugitives. Meanwhile, Hawkeye and Ant-Man accepted house arrest as punishment for joining Team Cap, while Bucky Barnes was secretly granted sanctuary in Wakanda by King T'Challa, the Black Panther.
In Infinity War, Steve Rogers led his team back to the American soil in response to the disappearance of Tony Stark after the Black Order attacked Earth searching for the Mind and Time Stones. At the Avengers Compound, the "Secret Avengers" came face-to-face with Secretary of State Thaddeus Ross, who immediately ordered them arrested by James "Rhodey" Rhodes. Instead, War Machine joined up with the former Captain America, guaranteeing himself a court-martial. Also throwing their lot in with Rogers were Vision and Bruce Banner, which brands both as criminals in violation of the Sokovia Accords. Meanwhile, Thor has never heard of the Accords, so he will no doubt be surprised to learn that as an Avenger, he's now also a fugitive by the humans' laws.
Avengers 3 leaves a huge amount of the fallout from Thanos killing half of all life in the universe up in the air. In terms of the Accords and the Avengers, the biggest question is, do they still hold? Secretary Ross is the hardline enforcer of that law; if Ross is dead, will the next bureaucrat with authority see the logic in allowing the Avengers to respond to this universal crisis? Or if Ross is still alive, will he finally relent and grant the heroes amnesty? Unless the law is changed or relaxed, technically, the surviving Avengers - Rogers, Romanoff, Banner, Thor, Rhodes, and possibly even Rocket (who's an alien) - would still be considered in violation of the Sokovia Accords if they leave Wakanda and return to the United States.
Since Civil War, the Sokovia Accords have been applied throughout the MCU, with the legislation also impacting Agents of S.H.I.E.L.D. and Marvel's Netflix series. Spider-Man: Homecoming even showed the law is being taught in high schools (Tony Stark could have registered Peter Parker to allow him to freely operate as Spider-Man).
But at this point, have the Accords outlived their usefulness? The issue of superhumans needing to be accountable for the damage they cause remains prevalent, sure, and there was always a logic to the Accords; it's there for the reassurance and safety of the rest of the people in the world. However, after Avengers: Infinity War where New York suffered its second alien attack and Wakanda was full-on invaded, it's clear they needed to be freely violated. There was no time to consult the UN when the Black Order arrived in New York (though Tony Stark has authority, Doctor Strange and Wong are not registered as far as anyone knows). Similarly, Wakanda was defending its own borders, though it chose to accept help from people the UN considers "war criminals". In this crisis, adhering to the Accords would mean waiting for a committee to debate and approve action, and there was no time.
Going forward, the MCU must address whether to keep or toss out the Accords as a story point. They were created as a reason to start a Civil War but that time is over. It's otherwise detrimental to have Marvel's best superheroes branded as criminals. Maybe, like in Star Trek, the Avengers saving the world eventually absolves them of their crimes.
If the Accords do remain in the MCU after Avengers 4, heroes like Captain Marvel may be better suited to operate within such boundaries, but being a superhero often means taking matters into your own hands. Therefore, it's probably time for the MCU to scrap the Sokovia Accords for good.