We all know the ‘default’ anime design; the template that’s often used to build upon and create a new design. Perhaps it’s sometimes not as simple as “change the hair and clothes and call it a day” but the basic concept is clearly there. Often instead of simply creating a new body type and face from the ground up, built to its core around the essence of the character being thought up, Yoshiyuki Sadamoto simply takes his usual template and makes alterations to it.
I believe that, beyond the usual focus on prettified idealism that anime tends to exemplify, this is also an attempt to keep a protagonist neutral and thus easier for viewer projection. While some artists specifically give their characters imperfections for the specific purpose of making them more human and relatable, a very common occurrence is anime is the opposite; not defining a character too much, keeping his design devoid of much unique anatomical or facial definition.
Of course, there are exceptions; a lot of Japapnese works don’t solely restrict these types of defining character features to its less-important characters.
A strange example that seems to try and mix both philosophies:
Look at that fat guy. In-universe, he’s not meant to be attractive at all; the entire point is that he’s a short, overweight character. And yet his design is as simplified and stylized towards a ‘cute’ ideal as much as possible. This is the moe-est way possible of drawing a fat teenager; it’s what you’d expect a Pixiv shotacon fangirl to portray Cartman from South Park like, if she doesn’t go all the way and make him look like a sexy super-thin bad-boy nazi bishie. It’s a design that’s trying to have its cake and eat it.
Another interesting aspect is the way that the stock design for a “neutral” male in anime is also the one used for “hot girl” characters, because it’s so simple and thus offers more room for cuteness, and less room for wrinkles and imperfections (which are more associated with older or more masculine characters). It’s why they could take the male protagonist from Hayate the Combat Butler, put him in a dress and make him pose sexily, and suddenly he was just as ready to appease male otaku as the rest of the female cast. They can swap him back and forth between viewer insertion and fanservice, both versions clearly directed towards the exact same viewers.
I will admit that there is a strange sort of appeal to this sort of ‘neutral’ design, which is why it’s still popular to this day; even I feel the need to draw such characters from time to time. It’s particularly fun in a show like FLCL, where they’re taking very standard anime designs, pretty much character design blank slates, and doing all kinds of crazy things with them. While the model sheets were quite standard, the characters never stuck to a single model; Teysuya Nishio’s Naota and Haruko looked nothing like Imaishi’s Naota and Haruko, let alone Shinya Ohira’s. It’s almost akin to internet artists’ tendency to all draw their own visions of a video game sprite.