I think I speak for everyone here - Niko had an extremely low choice in customization. All there was was three stores of dull clothing.
I would like to see the customization of San Andreas back, but more detailed.
Clothing:
Many different varieties of clothing, such as 80s, 90s, gangster, classy, poor, rich, average, gym, swim etc... Obviously not a store for each, but definitely more than in IV, maybe with customizable colors?
Weight:
I would like to see this return, and be able to be fat and slow and unable to jump if you eat too much, maybe with some extra clothing options once you reach a certain level of obesity.
Muscle:
Ability to work out hard and achieve results, but not as easily as in San Andreas where you literally had to press some buttons repeatedly to get fully ripped. And muscle to power ratio.
Tattoos:
More customizable than in San Andreas, like choosing "arm" for example and moving it exactly to where you want it on the arm, with multiple tattoos allowed on each body part, along with different colored inks available.
Hair:
Different hairstyles, and color options for each individual hairstyle. I would also love to see customizable beards. Maybe some form of hair growth, at least in facial hair, but not too much as to turn the game into the Sims.
That's just a short list of customization that I'm looking for, IDK about you guys. This is not about vehicle customization, so don't bring that up and say I missed it.
What else is there? Can you add upon anything?