Whats the issue, Howard is a truly appauling actor, Cheadle's done some crap but he's a ridiculously better actor than Howard.
As said anyway, Downey carried the film and it was mediocre at best, a follow up film will have a little less backstory to work on and they can do more action and him being in suit which should make for a better superhero type film in this case.
I can't believe anyone thinks Terrance Howard is a good actor, I can't remember what the film where he was playing a rapper was called, embarassingly poor, and I'll forever remember him as being crap as Ali in probably one of his first few films. More than the bad acting his voice is incredibly irritating.