It's possible to replace the default Maxis skin textures.
It's possible to replace the default Maxis eye textures.

What about hair?

I don't like using custom hair because it makes the genetics all weird, but the default Maxis textures are so cartoony compared to my skintones and eyes.

I'm assuming it's probably possible to replace the default Maxis hair textures but I'm not equipped enough to try it. I wouldn't even know where to start.

But I'm sure there are some members out there who would know where to begin.

Anyone who's interested, let me know. Or if anyone would be so kind as to direct me to a starting point, I would be extremely appreciative.

XPTL297 at InSimenator has made a whole set of default Maxis hair texture overrides. I'm not sure how, exactly, s/he did it (because I haven't downloaded it), but I imagine that it works a lot like the overrides for eye colors and skintones-- either changing the material definition to point to another texture, or placing the new-default texture in the downloads folder with the same name, group, and GUID as the original.

(By the way, if the wonky genetics of custom hair bothers you, you could always colorbin it-- when a Sim is created with the custom hairstyle anytime after it's been binned, they'll have the proper genetics to pass on to their children. Here is a tutorial on changing the files so you can do that.)

acherubini, I find the 'custom' genetics wonky also

For skintones, I actually just use default replacements and I'm happy with that.
For hair, I just create my sim with binned hair (custom or not doesn't matter, long as it's binned, not showing up in the custom area). Long as it's binned hair when they are created, it doesn't matter what I do after that - changing hair in the mirror will not affect their genetics - I can make a blonde sim's hair purple, and their child will get the blonde gene. So it's only the initial creation that matters with regards to dealing with custom hair.

oh i know, i mean i hate the default maxis textures on the NPCs and whatnot and would like to replace them so i don't have to download and bin a ton of custom hair, ya know?

Good point. Fixing the townies up more would be nice, lol