I've been saying this FOR YEARS!!!
I'd also add the caveat of having the Valeria Richards relationship in the movies as well. In my opinion, the best way to do it, without the multiversal craziness, would be to have Doom have a brief relationship with Sue (before she falls for Reed), while in the states, and that's how Val comes into the picture (yes, making her older than Franklin), and when she gets older she finds out who her father, and travels to Latveria during his Civil War.
That's another thing, I'd change the Civil War from him walking into the place, and taking over, to him galvanizing the people around him. Just a way of showing how much he got the people to love him.
So yeah, Latverian Civil War, Valeria, and Kristoff, along with his backstory, are the best way to add layers to Doom's character, before introducing the Fantastic Four.