Confirmed: Drax, Mantis and Groot
One of the most shocking aspects of the end of Avengers: Infinity War was that Rocket became the only surviving member of the Guardians of the Galaxy, not including Nebula. However, in order to keep the franchise alive, it only makes sense that most of the fallen Guardians would come back in Endgame. Drax, Mantis and Groot are all confirmed to show up in the Infinity War follow-up.
Rumored: Star-Lord, Doctor Strange and Falcon
Star-Lord, Doctor Strange and Falcon are currently the only main heroes to turn to ash in Avengers: Infinity War that are not confirmed to return in Avengers: Endgame. While it certainly seems unlikely that we won't see these heroes come back, considering that nearly every other "snapped" character is definitely coming back, we still have to wait for that final confirmation from Marvel before we can mark these characters' returns as definite.
Confirmed: Harley Keener
One of the most surprising characters to return in Avengers: Endgame is Harley Keener, the young boy who helped give Tony Stark shelter in Iron Man 3. It's unclear exactly how we will see Harley enter the picture in Endgame, but we do know that he will show up somehow, maybe even to set up later appearances as a member of the Young Avengers.
Rumored: Red Skull
Another character rumored to return in Avengers: Endgame is the Red Skull, who briefly appeared in Avengers: Infinity War after being absent in the franchise since 2011. Red Skull's rumored return was hinted at back in August when the Russo brothers made a comment about the character during a Vudu Viewing Party, saying "Red Skull is free to leave Vormir, and he is also free to pursue his desire for an Infinity Stone." Will his pursuit lead him back to the big screen in Avengers: Endgame? Only time will tell.
Confirmed: Pepper Potts and Happy Hogan
Tony Stark's entourage is confirmed to return in Avengers: Endgame. Pepper is sure to have a large role in the sequel, as leaked merchandise and even a set picture has revealed that she will finally be suiting up as the Iron-Man-eque hero Rescue in Endgame. Happy Hogan, Stark's former bodyguard, is also confirmed to return, more than likely alongside Pepper.
Rumored: Sharon Carter / Agent 13
Sharon Carter, the niece of Peggy Carter and romantic interest of Captain America, first appeared in Captain America: The Winter Soldier and later returned in Captain America: Civil War. She is now rumored to return in Avengers: Endgame, possibly to once again serve as a romantic interest that will complete Captain America's story arc.
Confirmed: Hope and Janet Van Dyne
The two Wasp's of the MCU, Hope and Janet, both turned to ash last year at the end of Ant-Man and the Wasp, but are also both confirmed to return in Avengers: Endgame. Presumably, Marvel has big plans for both of these characters going forward, seeing as Wasp was an incredibly important member of the Avengers in the comics.
Rumored: Hank Pym
The only unconfirmed returning member of Ant-Man's party is Hank Pym, the original Ant-Man. However, considering that both Hope and Janet are showing up in the movie, it seems quite unlikely that Hank won't make an appearance as well, assuming that Michael Douglas is up to reprising this role yet again.
Though he was a villain in the comics, M'Baku has become a hero in the MCU, and is confirmed to return in Avengers: Endgame. Considering that both T'Challa and Shuri quite literally bit the dust in Infinity War, it's quite possible that M'Baku may become Wakanda's new king in Endgame, seeing as there are few characters left with royal blood that can take the throne.
One of the most least likely characters rumored to appear in Avengers: Endgame is Wolverine. Over the past few months, a number of rumors have surfaced that Wolverine will make his MCU debut during the post-credit scene of Avengers: Endgame. While this is currently not possible, seeing as the Disney-Fox deal has not yet been finalized and Disney cannot legally plan or incorporate the other company's assets until everything is finished, the deal is expected to be finalized at any point in the next couple months. Assuming that it's finished a bit before the release of Avengers: Endgame, Marvel will surely have enough time to put together a quick post-credit teaser featuring this particular X-Man. The recent rumors that Disney intends to cast Wolverine as soon as the Fox deal is finalized seemingly backs up this rumor.
Though he only was in Avengers: Infinity War for a short amount of time, Wong is confirmed to make a return in Avengers: Endgame. Presumably, he will once again have a small role in the movie, more than likely with Wong "dipping" out of the fight like he did in the last movie in order to protect the Sanctum Sanctorum.
Confirmed: Stan Lee
Unfortunately, the late Marvel editor Stan Lee only has two confirmed cameos left in Marvel movies, the last one being in Avengers: Endgame, as revealed in in interview with the Russo brothers on Comic Book via BBC Radio. Fox has confirmed that he will not appear in X-Men: Dark Phoenix, and it's still unclear if he will show up in Spider-Man: Far From Home, meaning that Avengers: Endgame could be the last time that Stan Lee ever shows up on the big screen. Hopefully, this final cameo will be one to remember.