Character design has been always something that I

Yes, i agree, it's hard. I don't know,... but i think maybe the right first step for creating a new character should be creating it's story, for that should tell us everything about how it looks like. But doing a story for every character we make...
Anyway, I think you're doing it really great because you have characters with a great variety of facial features, body types, different ages. And your work with expressions is great as well. Have you tried taking a character from a book (one that hasn't been made into a movie) and tried to draw it?