Immediately this is how I felt, but I think theres a reason RDJ was cast. The Russos have said that fhere is a reason rdj was cast as doom and its not his acting skills. We dont have the answer, but theres something with Tony Starks sacrifice that causes this Doom to do what he does. Whether this Doom is a variant of Tony Stark or this Doom just coincidentally looks like Tony Stark, something about Tony's sacrifice made this rdj version of Doom and the Russos and feige are confident in their casting of RDJ as Doom. So I think when the movie comes out that feeling you have and I once had will be settled a bit.
We might not even see dooms face in this movie either. Could just be downeys voice and acting behind a mask the whole time.
Whatever it is though I dont think the Russos will let us down. Them and feige know what's at risk with this movie and I think RDJs casting is way more than just bringing us a familiar face afain.