Dr. Doom actually held a mantle as Infamous Iron Man, just saying. And Dr. Doom being European and so not an American asset would better explain why the US government empowers Osborn since they need to level the playing field if the next Iron Man genius in the MCU isn't American. Compounded with the revelation of Wakanda. In the MCU, USA isn't the most powerful country anymore, and considering that USA in real life in response to Sputnik invented ARPA to ensure that America would never be "technologically surprised", it entirely fits.

All this stuff about Osborn Dark Reign coming into MCU has him playing a role that is better served by Dr. Doom any day-of-the-week.

And again there's no reason why Doom needs the Fantastic Four to work. Like you know the 1984 Secret Wars had Doom being taken down by Captain America with Reed basically being Cap's sidekick in that story and having no real interactions with Doom in that story. Doom's greatest story Triumph and Torment doesn't even mention Reed or the Fantastic Four by name once in that entire story.

So you can do Doom without the FF.