Kevin Feige recently held a big Q and A in Reddit, which got a lot of fans excited for obvious reasons. This is THE head of Marvel Studios after all so knowing that he would be open to any and (almost) all questions got a lot of us geeks pumped up. Fans were not disappointed with the Q and A, with one particular question getting answered teasing a big villain for the MCU.
When asked if the real Mandarin teased during All Hail the King would ever come to the MCU, Feige simply said "yes." That is huge, though it has a lot of fans confused since the villain is an Iron Man villain and Tony Stark is long gone (though he might be an AI who helps Peter during Spider-Man: Far From Home but that's just some fun brainstorming).
Depending on who you ask, the reveal of who Mandarin was in Iron Man 3 was either the best twist of all time or a letdown that soured fans with the whole film, despite a cool climax. Knowing that it was just an actor who posed no threat whatsoever resulted in a huge anti-climax but All Hail the King, a short film released straight-to-video, fixed that problem by revealing a very real Mandarin.
Fans interested in All Hail the King can check it out through the Blu-Ray release of Thor: The Dark World. We aren't sure when The Mandarin will appear in the MCU but it should be worth the wait.
Avengers: Endgame is now showing in theaters.