As has already been pointed out numerous times, there are several mods which can be used to change the looks of the characters (and objects) in your game(s). There are also several highly skilled modders in our community who delight in filling specific mod requests.

Also, as has repeatedly been pointed out elsewhere, it would require massive programming to build diversity into the games. While it is possible, it would drive the cost of the games far beyond what the price point they have now. It would also greatly prolong development time.

In other words, it is possible, but not necessarily practical for the format of these games.