The cameo at the end of the FF movie is obvious. That was my first thought when the Doomsday bit appeared.
However; I sure hope this "source" has no idea what he's talking about because I hate that. "Just chalk it up to the multiverse" is lazy writing IMHO. Keep Dr. Doom as himself. Crossing him into Tony Stark is a terrible idea to me, and weakens who he is significantly. There is absolutely ZERO reason to have any relation to Stark just because RDJ is playing the character. If he's "such a great actor," he should be able to bring us an entirely different character as Victor von Doom than the one he played as Tony Stark - and, again, there's absolutely no reason Doom should ever be seen without his mask, which, if RDJ does a good job of it, will help audiences not even think of the character as "Robert Downey, Jr." but simply as Dr. Doom.
Doom is the greatest villain Marvel ever invented; why would anyone think it's a good idea to change him? Don't water him down. Jack Kirby and Stan Lee are probably screaming "NO!" from their graves at the very mention of this.