Well, seeing as Corvo was largely silent in the first game, there's no reason to consider him an icon of Dishonored.
You brought up Snake, why is Snake the icon of MGS? Because of his personality, because he endeared himself to gamers, because we were given multiple games to learn about him and come to understand him. We go through a Nature VS Nurture struggle with him in MGS1 (well, more of a lack of one as he was a clone of a supersoldier raised to be a weapon) and he spends the second game mentoring us by proxy through Raiden, telling us to critically analyse all information and to make sure we understand what we're passing on to the next generation, not to simply imitate our heroes. In the fourth game we see him coming to terms with the fact that his status of a clone renders him almost incapable of living a normal life, but there's no reason he shouldn't try.
A character being iconic comes with us sympathizing with them and what we can learn form them. It's much more difficult for a character to become an icon than it is for an object, like just for the sake of example I'll pull something out of my ass on the spot.
It was easy for the Starship Enterprise to become iconic, it has a unique design, the camera eyefucks it at every convenient opportunity, and most importantly it doesn't have a personality, it can't talk. It can't be annoying to viewers. But Gene Roddenberry had to put titanic effort into making Kirk, Bones and Spock iconic, these characters had to hold the attention of viewers and leave a lasting impression. That ain't easy.
Now as for the diversity thing, yes it's obvious that Arkane are trying to score the easy brownie points with a female character. That was obvious from the time it was announced that Emily would be playable. Anyone who tries to deny that will be getting that cheeky smile. You know the smile. The one that says "You're bullshiting me. I know it, you know it. You know that I know it. And I know that you know that I know it. So stop."
But I ask, even if Arkane is doing this just to score points (which they are), what does that matter as long as the gameplay and characters are good?
At least they're giving Emily a new array of powers. Why not have a different protagonist in every Dishonored? A new protagonist with new powers every game? There's not that much in the Dishonored lore that really points to Corvo as being all that important in the world beyond his assassinations. Sure he has the Outsiders mark, but as far as I know, Corvo having the mark doesn't prohibit anyone else from having it.