This year's San Diego Comic Con saw Marvel Studios return to Hall H, announcing the now titled Avengers: Doomsday will be directed by the Russos and will star Robert Downey Jr. as Victor von Doom. The former have proven they can handle the Marvel sandbox and could even bring order to the Multiverse, like they did with the help of physicists in Avengers: Endgame. Response to Doom has been mixed though, with only 20% of this site's poll voters loving it. As part of that 20%, I will try to explain why I think the move makes sense to me. Even if I already made my case once for the Kangs.
Why it works (or not)
It's no secret that it didn't work out with Kang and the MCU had to pivot to another villain. However, any villain needs build-up. Even Thanos appeared before Avengers: Infinity War. Now, there is no time to make the audience care for the coming threat, so a face that is already cared for is chosen: Robert Downey Jr. Seeing him be the ultimate villain, is something that immediately invokes a sense of ''oh shit, that's trouble'' and an interest to see why that is. It's not hard to imagine how a character with Stark's arc, who already pulled some villainous moves, turns into a villain altogether. Even if he thinks he's the hero.
Like Tony Stark, Victor von Doom is an ego-driven technological intellectual. Downey Jr. already portrayed the ultimate hero. and it would be a real shock to the Avengers (and the audience) that the ultimate villain is now wearing the same face. It gets the audience's attention, but the narrative reason has to be strong for the audience to stick around. One reason could be this Doom (possibly the one from Earth-838 out for revenge with his Ultron at his side) is trying to mess with the Avengers by replacing his scarred face with that of Earth-616's greatest hero. Because that's what Doom does, playing mind games.
Ofcourse, it's also easy to see this as an act of desperation. Reception to Kang and the MCU wasn't great, so a new route had to be taken. Recasting Kang would've been easy, but who's to say if that would've created any interest in the character that it was already severely lacking. Moreover, many comic book fans have pointed out Doom has vast, layered history that deserves more build-up. Now that is something the general audience (i.e the real demographic) doesn't care for, but they could still be distracted by Downey Jr. in role that isn't his most famous one as they aren't as versed in the Multiverse as those fans.
What it means for Doomsday
Regardless of what we think of it, it's a choice by Marvel Studios we have to accept. I wasn't a fan either of the casting initially, but when I started thinking about the possibilities my hype skyrocket. Although Feige nor the Russos confirmed where Doomsday would be based on (since there's no comic with that name), it could be base on Time Runs Out, the prequel to Secret Wars 2015. The namechange could imply that this time around the story is told from the perspective of Doom. And why wouldn't they with the most famousand one of the most versatile actors in the movie portraying him?
In terms of actual story, Time Runs Out sees universe after universe falling to Incursions. The Cabal purposely destroy worlds to keep theirs from being next, while the Avengers and Doom seperately look for a solution. It is Doom that discovers that the Beyonders are responsible, and uses their own weapon (a Molecule Man-bomb) against them to steal their power to create Battleworld and rule it as God Emperor Doom. These final moments could very well be what the titular Doomsday refers to, either as actual an actual doomsday (Incursio of the Multiverse) Doom warns off, or as Doom's Day.
In the movie, this can work with a small changes. The Council of Kangs can take the Cabal's place, with them easily being defeated by Doom early on to show Doom means business. The rest of the movie could focus on the Avengers trying to stop Doom, unbeknownst of his goals. In the end, instead of facing many Beyonders, Doom faces only one: the Kang from Quantumania that evolved into the Beyonder in the ants' pocket dimension. While Molecule Man doesn't appear in the MCU, the Scarlet Witch could take his place as the bomb in Time Runs out, while Loki takes his place as the battery in Secret Wars.
These are just a few arguments and ways I could see it work. As mentioned in various comments on this website alone, there are many things I haven't considered. One being what Kang Dynasty was initially about. Regardless, I think picking Doom works better in the prelude to Secret Wars than any story with Kang or the Beyonder. Secret Wars always needed Doom, so might as well pull him forward. He's an endlessly complex character, and one a phenomenal actor like Oscar-winner Robert Downey Jr. has the chops to portray. This is only my look on things though, so feel free to disagree in the comments below.