So despite already playing a Kree villain in Captain Marvel, Gemma Chan will now be Sersi as well in the MCU. Between Captain Marvel and Crazy Rich Asians, I haven't seen enough of Chan to say how good an actress she is, but she has a good look for Sersi.
So apparently Dane Whitman, the Black Knight, will appear in The Eternals movie and Kit Harrington will star in the role. Harrington is a big enough star that it makes me think Marvel has significant plans for the character down the line. Personally, I think the first Eternals movie has so much to explore in the Eternals mythology - a big cast of Eternals + Deviants + Celestials - that stuffing the Black Knight into the movie is too much, but we will see.