Mod The Sims user tedw, has proved that creating default skin replacement mods is possible in The Sims 4.
For testing purposes, I made a default replacement of the blue skin tone in the game, changing the blue for a purple color. It shows that default replacement mods can be made for things like skin. Hopefully replacements could be made to fix issues with some of the darker skin tones lacking warmth and some of the eye colors being too similar (there are two greens and two blues that are almost identical to each other). Obviously this isn’t the same as adding new skin tones or eye or hair colors, but it’s a start.