As far as what GW itself could do, I think what I'd like to see is a move away from all one-dimensional characters having their incidental character traits be homogenous.
When a character is painted, for instance, I don't think the default should be for that character to look like he or she is from English stock. Sometimes there is a good reason for a given skin tone - a space marine chapter's geneseed seems to influence skin tone, for instance - but if there isn't, then let the skin tones we see have more variation.
When a character is illustrated, same thing. If the rulebook needs an illustration of an Ecclesiarchy priest, for instance, I think that by itself is adequate reason to draw a man. I don't think "Ecclesiarchy priest" automatically encodes "looks like an Englishman," though, in the same way that it codes "looks like a man." Tech-priests, we already know from books, are female just as often as male - but you wouldn't know it from the illustrations.
When a character is sculpted, same thing. Sometimes the character itself incidentally encodes certain information - an unhelmeted Blood Angel probably shouldn't have dreadlocks, for instance, simply by virtue of being a Blood Angel. An eldar guardian, tau fire warrior, or human soldier don't have the same coding of physical traits. I really appreciated the balance of sexes in the recent dark eldar kits, for instance.
When a character is written, same thing. If the protagonist of a Black Library novel meets an incidental character who could be male or female, let them be female as often as they are male. If there is a good reason for there to be gender imbalance in the incidental characters, put that in enough that the attentive reader can notice it. After all, the characters probably do - you'd notice if you were walking down a street in Manhattan and there were literally no women on the street.
As for how to achieve these things corporately, that depends in part on corporate structure and part on the attitudes of the artists themselves. For instance, let's say you tell Jes Goodwin that the next time he resculpts the Tau fire warrior kit there should be the option to create X many female fire warriors per box. If Jes is the kind of artist who would say, "No problem, that sounds great!" then all you needed to do is tell him. If he's the kind of artist who would say, "No way, no armies in 40K should have female models except Sisters and space elves" (not that I think he is), then nothing will change without some corporate coercion.
I think, though, that this sort of incidental sexism and racism in 40K art probably doesn't exactly stem from the artists themselves. I think it more likely that they know they're being paid to produce 40K art, they feel like the people who consume 40K art will consume less of it if it contains fewer white males, and they feel like their corporate patrons value 40K art that people will buy over 40K art that takes the milieu seriously. So I think what needs to change is the messaging from the people who pay the artists: Hey, we want 40K art to look like it draws from the entire human race, and not the male half of England. If there is a fan backlash against that, we will back you. In general, I think artists are happy when their corporate patrons invite them to stretch themselves and/or take their corporately sponsored art more seriously.
For me, that's what this comes down to - not fighting sexism, social change, or even engineering better attitudes in the 40K community. It comes down to taking the universe seriously. I love 40K, and I think it has the potential to be much more mature than it currently is (or has been in the past). I would love it so much more if it was more mature. I think a lot of 40K is dreck, but I don't think that because I think the source material is dreck. It's because the people who make 40K art don't take it seriously, and this is one way in which they don't.