It's incredibly rare that anyone in the MCU gets a happy ending. In fact, any hero that may have gotten a happy ending could be counted on one hand. While Wanda and Loki continued this unfortunate trend for Marvel heroes, Sam and Bucky walked away with an oddly happy conclusion at the end of The Falcon and the Winter Soldier.
As WandaVision and Loki made clear, the Multiverse is well on its way. These two series laid a lot of groundwork to establish a solid foundation for Doctor Strange and the Multiverse of Madness. Yet, The Falcon and the Winter Soldier doesn't harp on the Multiverse at all. It begs the question of how Sam and Bucky will fit into the Multiverse, or come out of the events set to transpire.
In WandaVision, Wanda lost her two children and Vision (again). It was revealed that Vision bought a piece of property in Westview for them to build a life together, prior to his death in Infinity War. With everyone in Westview viewing her as a villain that held them captive in her own world, she also lost the chance to return to Westview peacefully. Fortunately, it wasn't all loss. By Agatha's hand, Wanda also came into her own as the proclaimed Scarlet Witch. It's the only thing Wanda truly gets to keep.
Similarly, Loki experiences a number of setbacks himself. He comes into his own throughout the series; there's no doubt about it. Like Wanda, Loki is forced to revisit the trauma he has experienced (or at least the trauma another Loki variant experienced). In an attempt to change his ways, Loki builds bonds with Mobius and eventually, Sylvie.
Unfortunately, Loki is betrayed by Sylvie when she ignores his wishes and opens the Multiverse. It's also revealed that when Sylvie pushed Loki away, he likely wasn't returned to his own timeline. Instead, it seams Loki was pushed into another universe where Mobius and Hunter B-15 have no clue who he is.
Bucky and Sam certainly experience their fair share of issues individually and collectively during The Falcon and the Winter Soldier. Both of them struggle with their identity and resuming normality after being blipped away for five years. On top of this, they are still grieving a strong friend and leader in Steve. Sam weighs his options with the shield Steve left him and he fails to make people around him understand his perspective. Combined with his personal family struggles, Sam becomes increasingly frustrated. Bucky struggles to have himself and the world see him as anything other than the Winter Soldier.
As a team, Bucky and Sam fail to forgive one another for the wrongs they feel the other has committed. Wrapped in some of their own fears and insecurities, they refuse to hear what the other has to say. When they're met with mutual enemies, they're forced to confront this and eventually overcome it. The Falcon and the Winter Soldier ends with Sam and Bucky enjoying themselves at a Wilson barbecue. Of all the Disney+ series, why did Sam and Bucky get the only happy ending?
A potential answer is that something far worse is coming for them. Sam and Bucky's happy ending was a mere fake out to make people (including themselves) believe that they are in the clear. Loki and Wanda experienced internal conflict, much like Sam and Bucky. However, Sam and Bucky had numerous tangible villains stacked against them. They faced the Flag Smashers, the Power Broker (who viewers now know is Sharon Carter), John Walker, and at some points, Zemo. Exceptions for Wanda include Agatha and Hayward. For Loki, the opposition between Sylvie and himself, the TVA, and at the end, Kang. For Wanda and Loki, their conflicts were meant to set up the Multiverse. Sam and Bucky's struggles appear to have nothing to do with that, so what could they have to face going forward?
Zemo fought on Sam and Bucky's side but also moved against them. The Falcon and the Winter Soldier ends with Zemo at the Raft prison, but prison has never stopped him before. After his first appearance in Captain America: Civil War, it was unclear if Zemo would ever make a return. Now that he has, it seems like he has the potential to turn up again. The birth of Zemo's military mask certainly makes it seem that Zemo's time in the MCU is far from over.
Sharon Carter is another familiar face who was always viewed as a heroine. Now that she's been crossed and left out to dry by the Marvel heroes more than once, it seems she's finally decided to get her due. Sam kept his promise and got her a government pardon and it seems Sharon is far from done with her Power Broker status. Off the grid, Sharon made contact with a number of shady characters. With government access, Sharon certainly has the capacity to cause quite a stir-up.
John Walker is one of the bigger problems Sam and Bucky will certainly have to face again. Injected with the super soldier serum, Walker poses a bigger threat than anyone may care to admit. Stripped of his Captain America mantle, Walker flew off the handle. Of course, his story didn't end there. Walker was approached by Valentina Fontaine who didn't give him much of a choice. Believing he's destined for more and dying to put his newly acquired powers to use, Walker will be used by Fontaine in some capacity. Right now, it's uncertain what that may be. No matter what it is, it doesn't bode well for Sam and Bucky.
Wanda, Loki, and Doctor Strange will have their hands busy with the Multiverse. With all the villains and potential angles the Doctor Strange sequel may tackle, it probably won't be dealt with in one film. Sam and Bucky have a long list of people they have to deal with on the Marvel timeline everyone's gotten to know and there's no telling what the Multiverse may bring. While Sam and Bucky got the happy ending in their series, the two are being set up for something far worse.