Sure but there are limits to the way your can change his looks without making it into a completely different character. How about making the Lone Ranger Chinese?
I do wish people would stop making these reductio ad absurdem suggestions, because they not only fall under that logical fallacy, but also under the fallacy of confusing a specific argument with a general argument. The whole point here is that there shouldn't be a simplistic universal rule. You should treat each case individually and make the best decision for that particular case. And there are going to be cases where the best actor to play a certain role is not the same race as the original character -- like when the best actor they could find to play the Kingpin was Michael Clarke Duncan. There are going to be cases where it matters that a character be a certain ethnicity, and there are going to be cases where it doesn't. There are even cases where it would be a bad idea to use the character's original ethnicity -- for instance, the Mandarin, who was a blatant "Yellow Peril" racial stereotype which it would be inappropriate and insensitive to recreate faithfully (Ben Kingsley, who's Anglo-Indian, is playing the role in Iron Man 3). Trying to reduce everything to some single universal policy is missing the whole point, and refusing to engage with the nuances and complexities of the issue.
Really, all I'm talking about here is keeping an open mind, taking each instance as it comes and not making pre-emptive generalizations in either direction. Yes, saying "Every character should be a different race" would be ridiculous, but saying "Every character must always be their original race" is just as ridiculous. As with most everything in life, the best solutions lie between the simplistic extremes -- and the best solution for one case may be quite different from the best solution for a different case.
To be completely accurate, they made him black first, then made him into Samuel L Jackson.
Well, not quite, since the Ultimate Universe version of Nick Fury was deliberately and specifically based on Jackson; Marvel got permission to use his likeness for Ultimate Fury in the comics, in exchange for giving him first shot at playing the character if they ever brought him to the screen. Which worked out pretty well for everyone in the long run.