So: Downey is Doom. Seems equivalent to casting Brolin as Thanos, with less of a time factor for Downey because a lot of what we see onscreen will just be full CG imagery, same as with much of Iron Man’s later stuff. And the vocal profile for the voice acting will be wholly different to Stark so realistically the only time you’ll see Downey is when he does press, which is arguably most of the reason for the casting given that Disney urgently need to recapture the media narrative over the MCU.