(Disclaimer: I do not intend to offend, the links I show here are just to serve as examples. Again, this is entirely my opinion ;; )
I'm a sucker for character designing most especially when fashion comes into mind. I prefer humanoid-looking characters more however as monsters proved to be difficult when I draw it. I don't think there is a right technique or method. People would generally pick out what they are most comfortable with the most and can unconsciously be inspired from one of their previous designs that they like. Hence, there are a pattern for some. It's mostly subjective.
I'd say I prefer not too overly saturation in designs, it's a bit off-putting. My works fall into the fantasy genre so there's attributes that are uncommon here but are natural to their setting. Research for inspirations, too.
(This applies to solely humanoid characters) When I'm designing a character, I'd ask myself "how would it go if an actual person were to dress like them?". props aside, is it convenient and functional? I say this because there are a lot of designs out there, particularly female characters- whose designs are well...
please help her. please
I think colors should be really considered, and while designing outfits that would fit the character is fun, there should be a limit (?) Not sure how to word this, but there's a thing such as an overly designed, the examples that come into my mind are Precure and some designs in the recent Danganronpa. Tenko's overly-designed, the color combination just doesnt go well 2
As long as I could tell the characters apart, I have no problem with body, race diversity etc.
For fleshing out characters and all, I suggest that you try answering fake interviews (there's a lot in deviantart) but as your own OC instead, if that makes any sense. It helps that you create short interactions with other characters. Small comic and all. They would eventually develop as you write about them.