Kit Harington has been cast in Marvel's The Eternals, and Game of Thrones actually predicted it way back in 2011. As revealed at D23, Harington is joining the MCU's Phase 4, with the Jon Snow actor set to play Black Knight in The Eternals, which is scheduled for release on November 6, 2020.
Harington's casting means that The Eternals is going to make for a big Game of Thrones reunion, as his former co-star Richard Madden is also in The Eternals, having been cast as Ikaris. Madden, of course, played Jon Snow's half-brother (really his cousin) Robb Stark in seasons 1-3 of Game of Thrones, although the pair only shared the screen in the first couple of episodes. Nonetheless, that was enough to showcase the strong, brotherly bond between them, and for Game of Thrones to predict their MCU casting.
In "The Kingsroad", the second episode of Game of Thrones' first season, Jon and Robb say goodbye to each other as the former heads for the Wall to join the Night's Watch. As they bid farewell, Robb tells him: "Next time I see you, you'll be all in black." Jon responds with: "It was always my color." Unfortunately, fate conspired against Jon and Robb in Game of Thrones: the former went beyond the Wall, and the latter, despite becoming King in the North, was killed at the Red Wedding, meaning that next time never came to pass - but The Eternals is going to change that.
With Harington and Madden both playing prominent characters in The Eternals, then that means they are going to be sharing the screen together at some point during the movie. And with Harington as Dane Whitman, better known as Black Knight, then that'll mean when Madden does see him, he'll be all in black (in fact, with the red symbol on Whitman's chest, the Black Knight costume does look rather Targaryen-esque). It's not quite a proper Jon and Robb reunion, but it's as close as we can possibly get, and adds an extra fun, meta-layer to the upcoming MCU movie, which is now even more of a must-see for die-hard Game of Thrones fans.
Harington's Dane Whitman/Black Knight isn't actually an Eternal himself, unlike Madden's Ikaris, so it isn't fully clear at this stage just how he's going to fit into the movie. Whitman did have a relationship with Sersi (sadly not being played by Lena Headey here) in the comics, so they could be utilizing that aspect, but either way, The Eternals should allow us to finally see the two Game of Thrones actors fighting together, albeit in a Marvel movie rather than on HBO.