There is a strange assumption surrounding Doctor Doom right now: because he is the MCU’s next major villain and is being played by Robert Downey Jr., he must automatically be a bigger threat than Thanos. Great villains, however, don’t work that way. There’s a lot of potential in Doom, but he hasn’t yet proven it on the big screen.
Thanos set an almost-impossible standard, as he did not just threaten the Avengers. He beat them.
Doctor Doom Needs More Than Robert Downey Jr. to Become The Next Thanos

The Avengers did everything they could in ‘Infinity War,’ and Thanos just kept on going. He beat Hulk, overpowered the heroes on Titan, and ended up gathering all six Infinity Stones. Then he snapped his fingers, and he won! Half of the lives were lost. That’s why Thanos felt different.
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Marvel didn’t have to keep telling us how dangerous he was. Thanos did not need to say anything; his actions spoke for him. Doom could be an incredible Avengers bad guy. He is a genius scientist, a powerful magician, and an authoritarian ruler who has a huge ego. In the comics, he has even attained godlike powers.
None of that means that the MCU version is more threatening than Thanos, though. A villain might have an amazing backstory and unbreakable armor but still not evoke the same sense of danger. If Doom just walks into a room, insults the Avengers, and shows another facet of his plan, viewers may feel underwhelmed. The real question is what Doom actually does.
The Sentinels Could Change the Entire Scale

Here is where the Sentinels may prove to be very significant. If Doom has an army of Sentinels, they won’t just be another cool visual to show in the trailers. They may alter the way his war against the heroes will play out.
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The Sentinels could outnumber the heroes on multiple fronts, rather than Doom engaging in a one-on-one battle with each of them. They may be able to make the Avengers’ own world a weapon against them.
It would make Doom seem less like a cosmic bad guy who wants to destroy reality and more like a mastermind who knows how to take down his opponents step by step. Thanos won because he was strong enough to complete the story.
Doom must show that he can come up with the same vibe in a totally different manner. Until he does, calling him a greater threat is less about what we’ve actually seen and more about what everyone hopes he will become.
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