From what is available from early AOTC plans it seems like Lucas was considering another Maul-esque villain (which eventually became Ventress, Kit Fisto, maybe a bit of Grievous too) as Palpatine's new Sith apprentice and Dooku might've been intended as a Tarkin type.

It looks like the Lee casting merged the two characters into one and the role of the action-heavy villain/Sith henchman is largely taken up by Jango in the film.

Vader I think was born out a similar idea, merging some of the separate villain characters together.