>>8908220AOTC artbook says she was developed to fill Count Dooku's role, before Lucas settled on Christopher Lee.
Bounty Hunter took all the art and made Komari Vosa inspired by it.
When the comics and novels were gearing up for clone wars content, they needed to have more than just Dooku for everyone to fight, so they went back again to the AOTC concept art and made Ventress proper.
Lucas would eventually do his own take on her for TCW.