Your art skills have to be professional quality with the ability to work fast in both color and black and white. You need to be flexible with making changes at any stage from sketch to final art. Your figure drawing skills need to be top notch. You need to be able to draw people of all ages, genders and types in movement and at rest. You need to be able to draw animals, architecture, costume, landscape and interiors. You also need to develop research skills especially for science and historical illustration.
If you feel that you need an artistic muse or that you often run into "artist's block" then the field of illustration isn't for you. Publishers aren't going to wait around for you to be "inspired".
You have to be willing to keep learning and growing as an artist through all your career. Don't depend on one style of illustration to keep your career going. Take classes, experiment with new media, expand your strengths and keep growing as an artist. Diversify your areas of expertise so that you can survive the vagaries of changing needs and tastes in illustration.