Why would that be difficult? It's not as if women soldiers don't exist.
The examples that exist are Israeli soldiers and Kurds that I can think off-hand (both have women fighters). Do you really want to play a game mowing down women Israeli's or Kurds?
You'd have to create an entirely new universe, and it would not be a typical mainstream affair, it'd be a one off or its own world, a new franchise, and it would be notable, as an FPS, for mowing down women soldiers.
There are already games that implement these suggestions. However, there are also still developers who unconsciously make their games uncomfortable for some women to play. The idea is to get these ideas applied more widely.
Well, I'm talking about certain gameplay types that women are not interested in. According to SURL studies women prefer social while men prefer violent games, women prefer mobile games, while men play on PC. I don't think adding more women enemies in a non-sexualized way is going to make some specific types of violent games more comfortable or interesting for women to play.
I don't think that is the intent of adding more women enemies in a non-sexualized way, mind you, as I think the natural premise is to reduce objectification, which is perfectly fine, and no doubt that will be a trend in mainstream violent video games, if only because it would be a departure from the norm. I honestly blame the last decade of design decisions on 90s kids who grew up on Dragon Ball Z, don't be offended by the broad brush here, but all the styles seem extremely similar. Excess everything, elaborate everything.
But, anyway, don't expect swarms of non-objectified enemy women attacking a you in an FPS any time soon, unless it's a mod or parody of sorts for political reasons. Replacing all the models in a COD game with female characters or something, to make a "statement" about how "disposable" enemy characters are in violent videogames targeted toward men. As an actual mainstream product? No way.