GTA VI should have a female and homosexual protagonists.

[Qwark]

As OP said they don't have a reason not do it. But there also isn't a single reason to do it. Having a black protagonist appeals to black gamers of which there are more than a few.

If they want to add diversion and actually appeal a new audience, why not go for an Asian (they did with China town) or Hispanic instead. A GTA revolving around the opression of Mexican drug cartels does not sound all that bad. But I doubt a female protagonist will please the media. 

Besides are you assuming the gender of any GTA protagonist. For all we know Trevor identifies as a lesbian gender fluid trans male. It's improbable but still, don't assume genders.