A war between real-life Earth nations? That's a dark prospect for the MCU.
Thaddeus Ross being the villain is a thing that I am not sure it fits. An antagonist, yes, but not a full on villain. These politician figures are rarely evil in these types of films, unless they're a bit cartoonish, and I feel he will just be a guy who makes questionable decisions, upon the discovery of this new element.
Now as for Thaddeus Ross being president... that would be a bit complicated for the timeline, no? We already know there's a president Ritson in 2025, so he would be filling the role of president from 2024 to 2028. Three things that this could means: this can mean the movie is placed before the election of Ritson, so before November 2024 (though I now realize that would be impossible, given Ross was snapped, so he couldn't possibly fill that role, from 2020 to 2024... also he is just standing at Tony's funeral like he was a nobody), or this could mean this is placed after Ritson's presidency ends, so 2028 at the earliest, or Ritson, for some reason (death) is replaced by vice-president Ross. Judging by the plot of Secret Invasion, an environment where Ritson could be killed during the Skrull invasion could occur, I feel that would be the most favourable condition, for the timeline, to conciliate Ross being the president.
If Ross has such a central role, I can now definetly see why they decided to recast him. However, judging by his role as "evil" president, I can't help to think that Ritson himself could fit that role just fine. He has expressed desire to take offensive actions against Wakanda, which is surreal... and all for Vibranium.
As for them wanting materials from Tiamut, that is an amazing way to use that event to benefit the plot of a movie that's otherwise completely unrelated to Eternals. That said, and again concerning the timeline, Tiamut has been above the ocean for over two years now. Would it really warrant that much time until there is a conflict surrounding the resources that celestial provides?
I too think this is a fascinating premise for the film, but I thought these questions were relevant.